Seaming shoulder seams

I need to seam the bound-off shoulder seams of a child’s sweater and am wondering the best way to do this. I know to use the mattress stitch on the side seams, but what about a bound-off edge?

The best seam for shoulders is usually 3 needle bind off. Since you’ve already bound them off mattress seam would work. Or you can take out the shoulder find off and use the other.