It depends if you are quilting it yourself on your domestic machine or sending it to a professional longarm quilter... If you are doing it yourself then it will be fine to have a backing that fits your top perfectly, you'll just want to pin/baste it really well to ensure that it doesn't shift on you.
If you are sending it out to be quilted, most longarmers need a minimum of 6-8" wider & longer than the quilt top. Depends on the size of the quilt & how square your backing truely is. I would get a coordinating fabric & work it so that the backing has the same amount of it on either sides (or top/bottom) so that the quilt can be centered on it & then it looks like it was meant to be that way.
Hope this helps.