It always comes down to preference..... but that said, if I really, really like the pattern I will buy what the pocketbook allows. I usually buy in the marked down section of LQS, so that would mean at least 2 yards if not what is left on the bolt. I know if I don't get all that I can then I won't find it if I need more. If it is just a 'nice' piece then I usually buy 1 yard. When the stores have FQ's on sale I will buy them, but usually I don't because sale yardage is cheaper.....
Good luck building your stash! Oh, and when you go to make your quilt with your stash...... you will find you will need to make a trip for another piece to match..... never fails.....