I sometimes use sheets for backing if I am going to machine quilt it. I won't use them for a quilt that I'm going to handquilt because of the tighter thread count. But for a machine quilted quilt...go for it!
I like to look for sheets at thrift stores...if they are still in good condition and a pretty, but fairly neutral design, I buy them! I want to make sure they don't look have a "dated" design, etc. I prefer something like a striped or a pretty floral design.
Where else can you get a quilt backing for $2-4???