Just finished the sandwich
 

Do I trim the back and batting to the size of the top now or after quilting?

I always do it after... it may shift a bit depending on how much or how little you quilt it.

After quilting. The excess backing can be attached to a frame (if you use one). Also, the batt & backing can be taken up a bit when quilting.

After quilting~ things can shift~~

Definitely after. You might have to square the top a little plus like the other comment, it may shift or draw up a little.

Thanks, I always know I can count on this board for help

After. The front and/or back may pull up during the quilting process.

I leave about 1 1/2 to 2 inches around the edges of batting and backing. Then after quilting, I trim completely.

I trim only when the quilt has been quilted just to be safe.

Always after for me.