it depends on how the quilt is to be quilted/and by who-
if sending to a long armer they have their own specifications- i generally ask for 5" all the way around-
the backing/batting is trimmed even after quilting- and no reason to think of it as waste- it is very usable fabric that you are cutting off- putting back in your stash for another project.
without excess all the way around you can wind up having to cut your top after quilting-
it is needed to attach to the frame- and to compensate if everything is not quite perfectly square-
as you quilt the fabric is drawn up (like it is when doing applique and embroidery- that's why those squares are always larger- then trimmed to size after the applique/embroidery is finished-
the more quilting the more the fabric will draw up- meaning the more backing/batting is required to have enough for the top-
it is very (traumatic) to be quilting a quilt and get to the bottom and find you are 2" short---find you've run out of backing with 2" of top left to quilt---better to have excess to trim than be short!
and the more you have all the way around the larger your pieces left over will be- 1" might not be that usable- 4" lots can be done with...
but every quilter is different- and requires different amounts to 'make sure' there will be enough