My general practice regarding stash is to buy what I like - colors and patterns that I am drawn too. I steer clear of "other peoples fabric" as it is almost never what I would choose. I don't have the space for fabric that is not my taste, and it ends up just sitting there. Free or estate sale/yard sale fabric for me is not even tempting. I do, however, haunt a few thrift stores for good quality sheets. Those are often my favorite backing fabrics.
I have a good friend who sews at my house a lot, and her taste is very different from mine. You can look at the color stacks on my shelves and know exactly what I bought or what she bought. She uses mine all the time - I rarely use hers.