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.

I do trim it up to 2-4" to count down on the likelihood of it getting caught in the quilting. Has happened before.
After quilting I trim it even with the top.

It's not just for a shifting top, you also want to have something to hold on to when you get to the edges.

I wait until after. However, I do not make it as large as instructed. I usually only put about 2-3 inches on a side.

I always do after, and there's always some shifting to be done anyway.

Leave the surplus on until all. The quilting is done. Then trim ready for the binding. You may find you trim a little off the top as the edges have become wonky during the process of quilting. This wonkyness is normal.

Joe, remember to stitch about 1/4" around the top when you are ready to start quilting because it will keep the batting from raveling, if you are going to hand or DSM quilt it.

I always trim after. I also stitch around the edge to hold the three layers together before I trim.