I don't like patterns so I rarely buy what a pattern says I should buy. I like to just let the fabric tell me what to make with it.
I buy sets of coordinating fabrics that I like. The more I love the fabric, the more I buy - lol.
I mostly buy half a yard each of 3-4 coordinating fabrics.
If I am getting a great price or it's something I've been looking for I buy a yard of each.
If I see one fabric that I like with nothing coordinating, I try not to buy it, but if I HAVE to have it, I buy 1 yard.
I like precuts, but I find I don't really have a use for them unless I buy specifically for a project.