My Longarmer prefers a 1/2" seam pressed open. I also use a tiny zigzag. I think the zig-zag was recommended by Linda Taylor on one of her videos.

Never thought of using a small zigzag. . . huh

1/4 and press it open.

If you are going to send out your quilt to a LA'er, please don't make a flat-felled seam on the backing....it's way too heavy to sew through and expect everything to stay lined up and pretty....and no, I won't tell you how I know.

I use a 1/2" seam allowance and press open. I am a longarm quilter and that seam is the flattest one that doesn't deflect the needle as much as the 1/4" seam seems to. Pressing to one side creates a lump that goes all across the quilt, deflects the needle and I really hate them! If a customer quilt comes with a backing seam pressed to one side, I have to press the darn thing open so the quilting will nice and there won't be a lump.

I usually use the 1/2" seam and press open. The zigzag seems like an interesting idea.

As a long arm quilter I'd prefer the 1/2" to 1" seams pressed open. Zig zag would be ok if small enough you can still press open. Also a shorter stitch length is ok if it does not cause puckering of the fabric along the seam. Th critical thing is can you easily press it flat and have it stay that way. If you have difficulty pressing it flat you are likely to get back a quilt with some small folds stitched in.

I use a 1/2" seam.

Usually a 1/2 inch seam works fine for me.