I make a number of the couch quilts.. throw size. But even then it varies... 50 x 60 to 55 x 70. I think that size works unless you have long legs
I have made baby and toddler quilts, and even those I vary sizes. I tend to agree with those who say the patter dictates. I have however let the backing decide the size... if I am making a couch throw size quilt and I am leaning toward 70" but my backing is 60" .. I stop at 60". I see no need to piece the backing for 10" I think its whatever works best for your situation