I do mostly throw size (bigger than lap but smaller/shorter than twin). Usually 4 yards of regular width material is enough to cover the short side (2 yds = 72 inches) and 40* 2 is enough to cover the length... However, I hate having just a plain seam so I often insert pieces/blocks or strips. Most of the time they end up somewhere around the middle unless I plan when layering. My backing is usually wider than my front by a good few inches so I try to align a seam from the top with the seams on the bottom on both sides. It actually gets it pretty darn straight. I don't often take pics of the back, so I don't really have any examples to show.
On my last quilt which was King-sized, my 108 inch wide fabric shrunk a good 6+ inches and was too small. In that case I pieced a width of a different material and ran it across the top of the quilt. What's great is this now allows for a pretty "turn down" effect on our bed. It was a "mistake" that turned out great and I am tempted to do it for any bedquilts that I do in the future!