I'm sure you will get many answers...but for me, if it's a larger print that I think will be good for a border, 2-3 yds. For anything else, I often only buy 1-2 yd. I have a bunch of FQs from when I first started building up my stash, but often want more than that. If it's a favorite color that you think you might use often, and a good deal comes along, you might want to stock up. I use a ton of blues/greens, so I don't mind getting a little more. Sometimes it goes in the top, sometimes in the back.
I guess it may also depend on your style...if you like scrappy quilts, smaller amounts are fine. If you prefer a less scrappy quilt, try to get a couple of yards so you have enough for whatever pattern you may have thoughts about using it for.