I think it's going to depend on what size you find you like to make best. I make quilts for charity-kids and soldiers, so if it's a kid's fabric I try to get 1-3yrds, for soldiers 1-5 yrds if it's a great price.
Basics that I know I'll use-like red/white/blue or patriotic patterns, neutral tone on tones that will go with a lot of things, I try to get as much as I can afford-I have a yearly budget for fabric/supplies and keep it in cash in an envelope. Keeps me honest that way.
Some advice from one who has way more fabric then needed....you don't need to have an entire room of stash. I have a friend who has 3 closests full, and some hidden up in the attic because they remodeled.
Think before you buy, will I really use it? Do I absolutely love this fabric or am I just buying it because it's so cheap and I'll find a use for it sometime?
I've started only buying fabric when I could find enough to do an entire quilt when super cheap, because in the past I would just buy everything in site when it was cheap. Now I have a lot of fabrics that I just can't figure out what goes with it.