It depends on the quilt I am making if I use the wide backing material or the regular 45 in fabric. Last fall I made a King sized quilt for our bed and it has a one piece backing. I love it. I recently made a queen sized quilt that will be auctioned and it has a one piece backing. When I work on smaller quilts, like the baby quilt I just finished this week, I actually used the rest of 5 inch squares not used in the quilt top and pieced them into the back. I only had 10 squares left and would not have used them for anything else.
The thing I really like about the one piece backing is the look of it. So beautiful and I think much more professional looking.