Don't worry about 'building a stash' because over time it will happen.
Another example of how a stash is created: You're working on a project and when it gets down to cutting and aligning all the fabric together in one spot, you may not like a certain fabric choice you made so what do you do, keep it and work with it or go buy another piece of fabric. Or, you reach the end of your cutting and gosh darn it, you simply did not have enough of a color so what do you do, go back to the store and try to match it up to color.
It's still your personal choice; but eventually you'll find that you like the look of in certain fabrics, say civil war or Aunt Betty's or Batiks, those you will always look at regardless of what project you're working on and purchase it if it makes you happy to have a piece of it because you know you will use it.