I'm getting ready to sew on a 2 1/2" binding. What seam allowance should I use?

probably about one quarter inch - maybe a little more if you want to take the chance of not covering the first stitch line with when the fabric is wrapped for the second stitching.

I usually sew a 3/8" seam allowance, but depending on the batting, I do make adjustments.
I will baste for about 6", stop, and then wrap the binding around and check to see if it is where I want it. Adjust and then sew it all down with a regular stitch :D:D:D

yep. I'm like Amma

it depends on the edge of your quilt top too. are there any blocks that have points to the edge you can't cut off. if so then 1/4 inch for sure.

i never thought of testing that is a wonderful idea i usually sew a wide 1/4 inch :lol: :lol:

^^^That's good advice!