My own thought on fabric buying is: 2-3 yds if I know I won't use it in a border, 3-4 yds if there is a possibility that I would use it in a border and within the quilt (to tie it in together). If I really, really like the fabric I buy the whole bolt.
Most important consideration for my fabric shopping: all of this depends on my spare monies when shopping too tho. I try not to shop at all, unless I have plenty of extra monies. You never know what you may come across that you can not live without adding to your stash or future projects. ;)
When we go on vacation, I always set aside extra monies especially for fabric shopping.