I used to buy fabric ahead, but I found I did't have enough, wrong color, changed my color interest, the list can go on and on. A few years back I decided not to buy anything bigger then a fat quarter. If I decided on a project, then I would buy the fabric needed plus a little extra for mistakes.
My stash comes from left overs, fat quarters and inherited fabric from my Mom & sister that I use on backs or community quilts. I have only 1 cabinet full of this. I don't like clutter.