Window methods are harder to analyze, and I'm sure once an analysis is done they will be shown to be inferior.
I don't think a window method will incentivize avoiding the beginning of a round, but in general, allowing cheating by joining only long rounds is also something we wish to avoid. In my method, the operator's score is precisely balanced to make the joining time irrelevant. If it is decreased or increased, it will incentivize miners to join early or late, respectively.
From a quick preliminary analysis of the sliding windowing method, I don't see any disadvantages of the method. No matter what happens, I don't see any advantage of either joining or leaving right after a pool solves a block or long into a round. Since solving blocks are independent events, your payout for participating at a given point can only be calculated based on the pool hashrate for the past n hours and your hashrate for the next n hours.
I think a window size of 2 or 3 times the average time to solve a block would be good.
I've also considered random windowing, where a window of time x-seconds large, based on length of round, proportionally larger as rounds grow in length, is used. The randomness of the window could mean that at times, only shares during the first part of the round, middle, end, or somewhere in between may be used.
This gives incentive to stay during the entire round to insure you get paid (if you joined only during the start, or only at the end, you may be excluded based on the random start position of the window) and as it would be a proportional count of shares during that period, payout would be fairly maintained to your current per-block average, while eliminating any reasonable way to predict whether it would be good or bad to leave a round. Unfortunately, this does also leave open the possibility of having some honest and hard-working miners being excluded due to downtime, network outages, etc...
...FairUser and I have been having some lengthly discussions about this, and we're also taking into consideration many things we would like to find remedies for; the poor efficiencies of some users, fair payment systems, incentives to participate full-round, added incentives for block solvers, etc...
Hopefully we'll have something drafted up and can begin implementing our solution soon.