Hi JessieRose,
Prednisone is a very powerful drug. I feel a difference on a 1/2mg change in my dose. Like Lynnwood mentioned, she feels a change on 1mg. That's why it's so important to taper down very, very slowly. Even a small change in the dose will make your body react in a positive or negative way. So go slow!! Especially when you're entering the "low dose" ranges of 10mg or less. Small tapers of .5 or 1mg will make big differences to you.
Good luck on the Cellcept! I've heard really good things about it!!
I forgot to answer your last question... I find that I retain less water at the lower doses, not more. It could be different for each person though. The best way to combat water retention is to actually drink more water.