I know that some wash their fabric before starting a quilt. I always do myself, but I have never washed the batting. Should I be doing that?

I don't prewash batting, but I use W&N and it shrinks very little.

I use warm and natural/white and I do not prewash, but this does not have much shrinkage anyway. I also do not prewash fabric.

you can..BUT sometimes just hanging it up to FALL naturally is all it needs!

You can preshrink a batting if you do not want a crinkle look. But if you are using a polyester batting there is no shrinkage.
Preshrinking a batting is pretty easy if the batting has a good scrim( thats what holds the cotton fibers in place). I have had the best luck preshrinking Warm and Natural ( or warm and white). It has a great scrim, I soak it over night , once in a while submerging the batting as it likes to float. If I get lazy I wet a towel to use a a weight on top of the batting. After it is fully saturated with water , spin , toss in the dryer, regular cotton temp. Do reposition the batting so it does not tangle lke sheets tend to tangle. Once its dry you are ready to use it.
If you want a crinkle look just use it with out pre treatment. You may want to smooth it , if it was a folded packaged batting, by tossin in the dryer for a few minutes.

Thanks for all your responses! I knew I could count on my Quilting Board comrades!

ditto here

if you are worried about the batting shrinking (which really depends on the quilting done on the quilt) why not buy one that doesn't shrink? there are so many to choose from now- you can get ones that don't shrink at all- ones that shrink a little- ones that shrink alot-
out of the 150+ quilts i've made- i've NEVER pre-shrunk a batt- and i've never had a quilt not turn out because of batting shrinkage after the quilt was finished and washed-
and i always wash and dry my finished quilts on regular cycles in the machines- going to a laundry mat if the quilt is too large for my machines.

I only prewash if I'm buying it from someone else's home and I don't know where it's been. In general I don't prewash because I like the feel of the fabric. Never have prewashed batting but I too use W&N.