I think you gotta do what works for you. I am a stasher - if I see something I really like and I have the money, I buy a couple yards of it. A lot of times, I will flash vision a quilt with some fabric I see in the store, so often I have an idea of where I want to go with it, but sometimes I buy fabric just for the pleasure of having a piece I really like. Some of my best quilts have come from my stash - sometimes I have to buy fabric to go with what I have, and sometimes I can cobble something together without leaving the house or ordering anything.
I have a quilting budget that I set side because I went for many years feeling like I couldn't spend money on the things I really wanted. I don't spend all the money each month, but I probably get pretty close because I'm trying to build up my stash. I got so depressed when I felt like I couldn't buy any fabric that my creativity just dried up, primarily because I didn't have a decent stash to pull from - lots of focal fabrics, not a lot of blenders. Now I try to split my funds between the two a little more evenly. I seem to have a Kona cotton fetish I can't shake lately; I'm buying it by the bolt!