As some have said, I usually buy a bit more of each fabric needed for a specific project, just in case I cut wrong.
Leftovers gointo stash.
I've also bought lots of fabric on sale, half price. Of course, I bought yards and yards. I still have loads of it.
I would say it depends on what you like to sew. If you like mostly matchy-matchy, planned, only use a few fabrics-typequilts, I wouldn't spend much to build a stash. If you love scrappy quilts like I do,then buy on sale, colors and prints that you love. You can use them for years. Fabric kept out of sunlight,heat,cold,and damp will be fine for decades.