I learned the hard way to leave quite a bit extra material on the edges. After I quilted I had a corner that wasn't covered. I decided it would be good place for a label. The quilt was for my GS so I put a note on muslin and placed it over the bare spot.Walla, covered!

I cut my batting larger than the top but not as large as the backing so I don't have as much waste. The left-over backing goes into the scrap bin. Once I worked with a same-size top and back for a quillow and the back was off grain. It never went together right and looked like crap when quilted. So I'd rather have the backing oversized and allow for adjustments.

It's because when quilting the body of your quilt will shrink up.

I go wider also, the excess I cut off will not go to waste as I have a large enough piece for future squares... I don't like the 1-2 inch strips that I cut off my first few quilts.

I have the backing and batting afew inches bigger for squaring up after quilting. I love left-overs for scrappy quilts, and I use the left-over scraps of batting to stuff doggie and cat mats for the local no kill animal shelter.
pam

I think that is a WONDERFUL idea to use the batting for doggie mats at the shelter! We have a no kill shelter here in Maine near where I live. I am going to start doing that!

Those scraps are not waste, use them for scrappy quilts, I've made several Mile a Minute quilts with the long strips of scraps, that I have either given to family members or charities. They come out great. If you google Mile A Minute quilts you'll find some pictures, there are endless ways to use the blocks. My favorite is to alternate a white on white fabric block with the scrappoy block.
Also I save all the batting scraps & fabric scraps that just can't be used in quilts & stuff them into a simple denim or heavier type fabric pillow cover, sew the opening closed & donate them to the animal shelter for pet beds, the shelter loves them.

As well as all the other reasons mentioned, if you send it to a long arm quilter they need it for the clamps so it out of the way of the machine head and can go to the edge of your quilt. I round off/up and cut my backings total of 10inchs larger then the top. But I Love scrappy quilts:)

Many quilts shift slightly as they are being quilted, this will give it a little room.

I always make batthing and backing larger - I'd hate to get to the edge and not have enough of one or the other. I keep the 'leftover' backing for binding on the quilt or save it for another quilt. Works out well. The batting leftovers - well, I still need to find something to do with them - sometimes use them for table runners - as I have a whole bolt of batting and just make it big enough to make table runners...