More Help Needed in Sandwiching
 

I got my throw size quilt sandwiched with no wrinkles. I am doing the happy dance. But, boom, brain hit a brick wall. Do I trim off the excess batting/backing now before I quilt it, or after it's quilted? I'm working on UFO's and been awhile since I actually quilted something. Thanks for answering a basic question.

After is better imo.

I think you need to wait till you're finished. That's what I do and that's what I learned.
Quilting could make things spread out a little bit and you want to be sure there's something there.

I always trim after. Sometimes either to top or the back slips.

Generally after ... howeverrrrrrrrr .... it depends how much you mean by "excess"!

If the excess is excessive, for either or both,
you may want to trim some off before you start quilting.
Otherwise, the excess could create way too much bulk, be in the way and
all-too-easily fold over and get quilted into what you are doing.

i always trim after because the quilting can draw up the quilt top.

If I have a large amount of backing and batting. Then I trim to about 2-4", in case quilting takes up more.

What QuiltE said.

It's nice to have a couple of inches of backing and batting at least around the edges so you have something to hang onto while quilting at the edge and to also have a space to put a fabric scrap off the quilt to test your thread tension.

I would wait until later. You never know what may happen between now and then.