I think it depends on how many colors you like to use in a quilt and what size you typically make. I like lots of differnent fabrics so for me buying a yard is usually enought... but if its a great fabric on sale and is going to work its way into more than one quilt 2 yards. If you are a three color quilt kinda of quilter 3 yards or 3.5 if its going to be a focus and its going to be a bed quilt... king size better add another 1.5 yards minimum.
White on White or Cream on Cream prints .. I use tons .. so if it on sale ( a really great sale).. sometimes 6 yards or I have been known to buy whats on the bolt.