The most important part is to make sure you're not getting timeouts etc.
BTC Guild was terrible for this a couple of weeks back (although much better now). My rough calculations seemed to indicate that at points I was 20% down on what I should have been mining.
Make sure you've got two miners running per card with one pointing at another pool such as Deepbit (whcih I've never had bad problems with timeouts....)
As for the payout per block, yeap it's going down, but BTC Guild is now solving about 50 blocks a day, compared to about 10 a day when I first joined.
So it all evens out, as long as your not getting timeouts!