What I do:
When I layer the quilt I make sure that everything is as "square" as it can be - then I pin the layers together.
After it is quilted, I trim the batting and backing even with the top - so that the three edges are even.
Then I sew the three layers together about 1/8 of an inch from the edge with a longish stitch (about 6 per inch) so that I only have to deal with "one" layer when I apply the binding.
I use bias binding - and I baste the edges of it together so that it stays "nice" and one layer does not creep ahead of the other - also so that when I apply it to the quilt, then I only have to fret about keeping "two" layers even.
I cut a wider binding than most - but that is not relevant - i do a few "test runs" of about six inches using a longish stitch to see if the batting folds over to where I want it to - it's easy to remove a "basting stitch length sewing"
When coming to the corners - I learned this the hard way -
Stop Stitching Here - - -