Friends call me the puzzle queen, and maybe it's because I'm a sucker for punishment, but figuring out the pattern with available fabric is part of the fun of quilting. It's good to have enough focus fabric if it's being featured, but when one fabric doesn't quite make it, I go back to the drawing board and re-work the pattern to see what I can do to make things work. I will confess, though, this is less fun when I'm doing a backing. By that stage of the quilt, I generally want it DONE, and having to piece the back because I don't have enough of what I wanted to use can be a pain.
If I'm buying for a pattern, I always pre-calculate how much fabric is needed and buy slightly more, usually rounding up to the next metre.