I'm amazed at how many people trim first. The couple of times I did that, my backing all bunched up on me when I attached the binding. So I leave it untrimmed (I always square my top before quilting), attach the binding 1/4" from the raw edge of the top,and then trim all the backing/batting to that same 1/4".
Like some others, I do double-fold binding. Cut 2.5", fold in half, attach on the right side of the quilt with a 1/4" seam. And then fold to the back and blindstitch by hand.