I donate off and on. My wife no longer can donate, so I've tried to pick up my slack a little. (She got two false positives on the preliminary tests they do, and even though those were *false* positives, they'll no longer let her donate. She got notification of this the same week she got notification of having made the Gallon Club--nice.)
I'm due to donate again soon, but I would urge anyone with a potentially big race in the next three months or so *not* to do so. When I was coaching, I saw how a donation wiped out a good runner's entire cross season. (Because it's not just a matter of getting your blood levels back up to where they were; while you're waiting for that to happen, you're losing training time at faster paces, hence losing some race-specific conditioning.)
My advice is similar to that given above: If you have a definite end-of-season big race/meet, with no other biggie scheduled for several months thereafter, then you can donate a week or two after your season-ender.