I strongly suspect that what the free market will show as the optimal will be low deflation instead of low inflation that Keynesian advocate.
I strongly suspect (and
Hayek would appear to have agreed) that the optimum is zero long-term net inflation as opposed to low deflation or low inflation. Of the block chain currencies, I suspect that a currency with a minimum block reward (I suspect around 1 or 2 coins per block to maintain the spendable (i.e. a private key known to someone and thus having a price in something else that will result in exchange) supply at 20-ish million coins) offers the best chance to achieve this with long-term stable demand. Perpetually low deflation is almost certainly superior to perpetually low inflation, however.
Comparisons of block chain currencies with a known block reward for any (past, present, or future) given block in the chain and stable time per block* with commodity currencies like gold fall are not totally reliable due to the issue of supply elasticity. The supply of such a block chain currency has basically zero long run elasticity in that there's no way, long term, to increase the rate of production (increasing coin value can short-term increase the production by adding hash power, but eventually there's a point where there isn't enough computer/electrical power to keep outrunning difficulty). Fiat currencies are generally quite (if somewhat unpredictably) elastic as monetary goods go and commodity currencies are rather more elastic than such block chain currencies (at some gold value, it becomes profitable to try mining on other planets/asteroids/etc. or even to devise methods of nuclear alchemy; there is no BTC value that makes producing coins profitable when production becomes impossible). Accordingly, if there's a reliable increase in BTC value over time (which is to say deflation), which means that supply's long run rate of increase is lower than that of demand, then that increase in value can be expected to continue for an arbitrarily long time due to a general lack of negative feedback (this is basically what happens as something becomes monetized).
If a positive correlation exists between the number of spendable coins over a given time period and the number of spendable coins that become unspendable over that period, then the supply of spendable coins will be decreasing (negative rate of increase): if demand is stable then supply will increase more slowly than demand. I suspect that there is such a correlation (though it's impossible to empirically know how many coins are unspendable or which are merely not being spent... any attempt to try to adjust the block reward to directly compensate for changes in spendable supply is thus impossible), probably with about the same order of magnitude as 0.5% per annum (remember that a mature bitcoin will probably see somewhere around 1.5% of holders dying each year, barring people living to be multiple hundreds of years old).
Reasonably perpetual mild deflation isn't terrible. The high degree of subdivision allowed for (and little to no practical need for bitbills) makes the "OMG! A carrot will cost .000000004 BTC!" objection meaningless. The issue arises when holding (or "saving" or "hoarding": the three terms are exactly equivalent... lending is not saving...) has a predictably positive return. The optimal outcome in that case is to delay an exchange until the last moment before you couldn't bear to not have the exchanged item. Having to pay a fee (it can be reasonably expected that most miners will essentially stop including free transactions in their blocks when the block reward is zero) in addition to giving up the expected appreciation increases the incentive to delay. The probability of blocks without transactions (and thus without fees) increases and the price the network pays for hashing power decreases. The decreased price pushes out miners for whom the new price is less than the cost of a given hashrate and thus decreases the hashrate and difficulty (with the time between blocks increasing until the difficulty adjusts). Slower confirmation of transactions and increased chance of double-spends then decreases the utility of the block chain for trading with untrusted parties and tilts demand for use of the block chain more to long-term holdings and larger transactions between fewer parties with higher levels of mutual trust. The primary trade involving BTC then becomes exchange into day-to-day media of exchange currencies that are appreciating less quickly in value but offer their users cheaper transaction processing and/or less fear of fraud. At various intervals, significant balances in the day-to-day currency are exchanged for BTC (probably at fairly predictable spot rates most of the time).
A currency with a reasonably perpetual mild inflation isn't likely to gain traction as a day-to-day currency: actually decreasing value will need to deliver dramatic advantages over the mildly deflationary currency in transaction processing and credibility and I'm not sure it's possible for any currency (at least any block chain currency) to deliver that sort of advantage over BTC (a currency backed by large amounts of physical force
may be able to deliver that, given things like the Rule of Law etc.). The decreasing value will generally mean that what constitutes a significant balance above which it is unwise to keep funds in that currency will be rather low: this in turn means a lot of trading into the savings currency which increases transaction costs. Mild inflation of the engineered variety is also generally self-accelerating.
A zero long-run inflation/deflation currency (which can be accomplished with a currency in which its value stays (or is perceived to stay...) in a trading range), on the other hand is one that can gain such traction, IMO. A constant block reward with a reasonably predictable (largely due to coherent debasement curves in both currencies, though this assumes stable demand in both) exchange rate should ensure that the hashing capacity is there for credibility (if the exchange rate is that 1 coin buys 0.05 BTC, but you're guaranteed 2 coins (0.10 BTC) even if there's no transactions or you accept free transactions then you're better off mining for the other coins unless you can expect to earn 0.10 BTC in transaction fees) and also that miners that are willing to forego transaction fees (e.g. to allow for micropayments to work) aren't mining at a loss.
*: as far as I know nearly every proposed block chain currency has that trait ([Mervyn]KingCoin sets the reward to be a percentage of all coins mined ([theoretically] the Bank of England is legally obligated to keep the annual increase in consumer prices to a range (IIRC, the current range is 1-3%)), BitCoin has the reward asymptotically tend to zero before rounding to zero, TimeCoin has a constant reward, etc.)