So the best strategy there is to pay it at the last possible moment.
It all depends if you expect the exchange rate to go up or down. During about the middle of this runup so far, I made a larger purchase from a Bitcoin merchant sooner than I had been planning on purposely to lock in the exchange rate such that if the rate dropped I got at least what I thought was a fantastic exchange rate. Ends up I had done that way too soon, but I'm not complaining.
But also what is available to use are derivatives such as PUT options at MPEX and BTC/USD futures contracts at ICBIT.
Now this adds a little more effort to a purchase decision but there's a good reason to consider it. Specifically, right now if I can wait until April 14th to spend my coins, I might get a small discount on a purchase then by instead today selling BUJ3 futures contracts on ICBIT at $150.
So let's say I am planning on spending 1 BTC, worth right now about $137.
There are buyers paying $150 for BUJ3 contracts. Each BUJ3 represents $10 worth of BTC/USD.
So by depositing 1 BTC into my ICBIT.se account, I would then sell about 15 of the BUJ3 contracts at $150 (quantity 15 was chosen since that is the closest quantity to the current sell price, for $10 contracts). Then it doesn't matter if the exchange rate goes up or down, I will have about $150 worth of bitcoins on April 14th.
Scenarios:
If BTC/USD settlement price on April 14th is $100, then you would end up with 1.410 BTC (after fees), worth about $141. Gain $4 versus making your purchase today.
If BTC/USD settlement price on April 14th is $170, then you would end up with 0.792 BTC (after fees), worth about $135.60, Loss $1.40 versus purchasing today.
Now this is just for holding off until settlement 10 days from now but in the scenario where the price drops I earned about a 3% discount for letting my bitcoin be tied up for ten days. Extend this out a few months where, for example, the BUU3 contract settles September 15th, 2013, then you have a higher price that you can sell at (currently $168, so you sell 17 BUU3s) and you get significant gains by committing your 1.0 BTC now:
Scenarios:
If BTC/USD settlement price on Sept. 15th is $100, then you would end up with 1.671 BTC (after fees), worth about $167. Gain $30 versus making your purchase today. (or about a 22% discount for tying up your money for a period of less than 6 months).
If BTC/USD settlement price on Sept. 15th is $200, then you would end up with 0.867 BTC (after fees), worth about $173, Gain $36 versus purchasing today. (or about a 26% discount for tying up your money for a period of less than 6 months).