I also use 1/2" seam for the backing. Isn't the reason for 1/4" seams in quilting because of the pieced blocks and the need for less bulk?
I find the 1/2" seam easier to keep flat. A lot of times I don't cut off the selvage - I know it won't have a problem with unraveling and the 1/2" seam takes it past the selvage edge.