I was wondering , when you piece the backing for a quilt ; should you trim off the selvage edge that goes down the center?

Either cut it off or clip the selvage at 4 or 5 inch intervals. When washed, it will shrink slightly more than the fabric and pucker the seam.

I always remove the selvage because it shrinks at a different rate than the rest of the fabric.

yes; otherwise it pulls; there is no give on the selvedge.

Yes!! Do a 1/2" seam, press open to reduce bulk. Keep the grain lines going the same directions so that you don't get surprises. HTH!

There should not be a seam down the center. You should always remove the selvedge.

I do remove my selvedges and often put a pieced strip into the center so it is not as obvious that I had to piece two lengths together to get the fit. That strip is usually right over center horizontally as I turn my fabric to get the 40 inch height (selvedge to selvedge) both above and below it.... Most of my quilts are close to 80 in length and around 65-70 in width, so 4 yards works really well for backing.

There is a way to piece so that there is no center seam, but if I remember correctly, you end up with two offset seams. I've never done it that way, becuase the middle seam/pieced part is now like a signature on my quilts!

I always tear it off.

My friend that taught me to quilt told me to always piece the back at least in thirds. Can't say that I always follow her advice!

I also piece in thirds