If you are having someone else quilt it, follow their directions for the back size.
Doing it yourself? Whatever works for you is what you should do!

I do need the 3 to 4 inch extra for the long arm as well. Needed so the side clips don't bump into the machine.

I learned something new today. I have never thought about using the trim away from the backing for the binding. Makes perfect sense. I am going to rent time on a long arm for the project I am currently working on. I will not begrudge the 4" recommendation for the backing size. Prior to this, I usually use a healthy margin and add the trim away to my scrap heap.

This past year, I've been using different battings in my quilts to see how they work, so when i'm done w/ the quilt, the extra batting(which is never much) is pitched. I don't want to combine a cotton batting w/ a cotton/poly batting into a project.

I quilt in a frame, so i also need extra backing to secure the quilt to the frame when I'm quilting the edges. sometimes i keep it, other times not. Again - it all depends...

If you are quilting yourself, then cut it to suit yourself.

HOWEVER, if you are having someone with a long arm quilt the top for you, they will need between 4 and 5 inches of backing fabric on EACH OF THE FOUR SIDES OF THE QUILT TOP. This allows for room to check tension, and for starting and stopping off the top so there are no "empty spaces" on the edges of the top. I personally ask for 5 inches on each side or 10 inches wider and 10 inches longer than the top.

When quilting it myself, only 1.5 - 2 inches as I spray baste. If send out it's at least 4 in. If my backing does not have that much, some scrap sewn to the edges makes it large enough for the gripers the LA person needs.

I agree with many - if you do your quilting on a domestic, you can get by with less than 4" around your quilt.

I quilt on a DSM and still like about 3". It gives me something to hang onto when I'm doing the border. 3" gives me a nice strip to cut 2 1/2" strips or squares for another quilt so it's a good width. I never lose anything in quilting except when the backing is fleece. Somehow that gobbled up at least an inch on each side.