Traditionally one would buy two lengths of fabric, split one down its length, sew the halves to each side of the whole, and end up with two seams running up and down somewhere on the sides of the quilt. The idea was to keep the seam from being in the center where the quilt would likely experience more stress from folding (and perhaps beard more).
Nowadays, though, there are no rules. It usually saves fabric to have the seam or seams going from side to side rather than up and down. You can also save fabric by using a diagonal seam. However, with patterned fabrics, it really doesn't matter at all to me.
Also, many people apply borders to the back, run a diagonal line of blocks down the middle of the back, etc. This can make the back more decorative and also allow for using smaller pieces of fabric.
For traditional quilts, I still prefer the original method, especially if it's a plain fabric on the back (such as muslin).