I wouldn't know how to sewquilt without a stash. I've never been without one.
I have started most of my quilts with fabric from my stash. I may or may not add to it with a trip to the quilt store, it depends on the pattern and who (if I know) I'm making the quilt for.
I tend to buy without a project in mind. If I see fabric that I know I will use and think it is cat's pajamas, then I buy 2yards or more of it. Likewise if I find fabric that I believe would make a good backing fabric (I don't always use muslin) I buy maybe 4 yards of it.