My true answer to your question is that in my 10years of quilting, I have done it both ways.
Nowadays, thanks to job losses + over the top medical expenses, we're now officially bankrupt so for the past 2 years, I've been pretty much broke. I can only buy fabric if I truly need some when there isn't enough in my stash to complete a pattern or border. I'll be going to Joann's soon with my Jan birthday gift certificate since I've run out of plain beige of all things. All I need it for is a narrow border on this pillow. That gift certificate will be used sparingly for only necessaries. I miss buying fabric just because I like it, with no project in mind for it. That was fun. Luckily, I have fun small gift projects my stash will support and some bigger WIPs for which I have enough fabric so I'm fine ...except for that thrill of spontaneity.
Oh, and I'm hesitant about even accepting a windfall of a big amount of free fabric like someone mentioned here (cleaned out someone's garage) since I may not have room if we must move to a tiny apt. Someday though...