I'm new here and loving this site!
I think that a lot depends on what kind of quilts you make. I used to make traditional quilts so I'd buy for the quilt, but now that my style is scrappy with a lot of applique I buy what I like that I think I'll use. Since I work pretty small quarter yards are usually fine, but if it's something I really like I buy half yards, and if I love it, (depending on the price), I'll buy a full yard or more. I also collect certain designers and fabric lines. I have Kaffe Fassett prints going back many years. I buy all of his stuff I can get, as well as my favorite designers at Free Spirit and Westminster. I love Tula Pink's stuff and buy smaller pieces because the larger prints don't work well with my quilting style. However, I love cutting them up and getting unusual pieces from them.
Stash building is ongoing. Since I collect for the long term I look for colors that are unusual and that don't come along that often. I do the same with patterns. I avoid too trendy looks because I ended up stuck with a lot of dusty country prints from the 80s and don't want to go there again! I've actually gotten pretty picky lately which is a good thing for my bank account.