I think it depends on what kind of quilter you are. If you are some one that uses patterns exclusively, then usually buying what the pattern calls for is more than enough. I have found that most patterns give yardage estimates on the "extra" side, so adding even more, would give more leftover fabric.
If you are someone that designs your own quilts, and want to be able to get up in the middle of the night when inspiration strikes and pull out fabric from your stash to start working, then I would definately buy fabrics in 1/2 yard plus range, depending on if it was a blender, or a focus type fabric. If you like kaleidoscope type quilts, where you need a certain amount of repeats, then you need to buy enough to give you the number of repeats you will need.
When I first started to quilt, and had a pattern that I was using, I would buy what was needed for the pattern plus at least 1/4 to 1/2 yd more. That way if I messed up cutting it, or had to unsew too often and ripped the fabric, I could just cut another piece with out having to go searching for more. And I always told myself, that any leftovers would help build my stash when I was first starting out. If I really liked a fabric for one of those quilts, I would buy more.
After I had been quilting awhile, I only bought extra fabric when I really liked it, or thought I would be able to use it in something else. I never cared for scrappy type quilts, so I wanted enough yardage to use in another quilt no matter what pattern I selected. But now I have so much fabric that isn't enough for yardage that I'm going to be making some scrappy type quilts.
Now, I try to not buy fabric unless I know exactly what I'm going to do with it. But if I find something on sale, and I think it will make a good backing, I will buy 5 or 6 yards, or what is left on the bolt. If I like it for doing a One Block Wonder, or Stack n Whack, I will buy enough repeats to make at least a lap size quilt. And if I decide to use it as a focus fabric for something else, then I will have plenty of fabric and would just need some blenders to go with it.