I like to use a print background if it goes with the front. However, I stay away from stripes and plaids -- if they are off-grain even a small amount, it doesn't look good. The quilt that I just finished had the backing and borders made from a black fabric with tiny white dots arranged in rows. I wasn't off much, but it bothered me to see my imperfection.
In general, seams go where needed to make the fabric on hand fit. I always pre-wash my fabric, so I can tell what the selvage will look like. I usually cut it off.
Dayle