So the three things to worry about are the drop in blood volume - but your body will replace that quickly if you continue to hydrate, the loss of red blood cells - which take a few weeks to replace (It's sort of the opposite of blood doping), and bleeding from the needle site.
So try to avoid dumbbell curls with the bike, and don't try to measure your V02 max in the week following.
No harm in hydrating well before the donation, I personally can't manage a full donation unless I have plenty to drink before hand. It just sort of dries up ... it's very embarrassing.