I try not to overthink, but am not always successful! While a friend needs the "Perfect" fabric for backings, I almost always buy backing fabric when I find good sales. If I can buy 4 - 8 yards at $4 or so a yard, I do it. When I have a smaller quilt, I look at the smaller of my large pieces, and for bigger ones, I check what I've got that's at least 8 yards. For some folks, trips to Goodwill or other thrift stores are what makes it doable.
And yes, I've got that stash... That's my investment for retirement - instead of using it sometimes, as it works, I'll need to use it as much as possible. one way or another, I'm hoping to be able to keep doing quilts that are donated.