I have never understood how you could sew to the back of the sandwiched quilt without trimming first, because how can you tell where the edge of the top is? But if you trim, and things shift, you could have a mess on your hands.
I recently saw a blog where a woman stitched all the way around the edges of her sandwiched quilt, less that a quarter inch. Then she trimmed, then sewed to the back, then pulled around to the front and stitched.
I have to say, it looked like it worked, made the edges flat and straight. But man -- sewing three times around the whole quilt? It's still worky, in my book!