It depends on your confidence and the size of the quilt. Lap quilts I use may just 3" larger but trust me, there is NOTHING worse than being short. The reason why you make the backing larger is because of the sandwich process. You put your backing down, then you put you batting which is usually larger. It is just easy when you put your top down, you don't always see the edges of the backing. I had a customer that made her backing of her queen size quilt the exact same size as the top. YIKES ! It took me twice as long but I was able to get it centered.