after quilting don`t trim your backing and batting flush with your quilt as it took me years to get a full binding leave a little as in this video i use Gale
http://www.mccallsquilting.com/mccal...ining_the_Ends

If you're using your domestic machine and not a long arm, trim off up to about an inch or 2. The reason is if you have all that fabric flopping around as you quilt, there's a chance it'll wind around and pretty soon you've quilted the flap up under your quilt. I tell you this from experience.....if I leave too much extra fabric on the ends, I always have to get my seam ripper out.

After - I like to have something to hang onto when I'm doing the border. I've never needed the extra to allow for FMQ, except with a fleece back. I pin about 4-5 inches apart.

After is best in my opinion.

Also, I do a really long basting stitch all around the outside of the quilt before I start quilting, and one horizontally and vertically down the middle of the quilt. I normally work from the center out on my small DSM. I spray baste so the quilt holds together really well.

Yep, after is the best time to trim!

How much excess are you dealing with? You should always leave a bit because things 'move' when you are quilting.
Better to be safe and trim when you are done. But, you don't need to have 10" hanging around and getting in the way when you are quilting the top.