Yes, you should have 5" or so on each side of the quilt, even more on the top and bottom. You can use a sheet if you prefer which you will not have to piece. If you have to piece, I would make the back in 3 pieces, going up and down.
Make sure the selvage is cut off the inside seams before you sew them or they may shrink and pull later on. Cut your one piece in two pieces. You could even make large pieces, like 20" or so. Good luck.