You can use mattress stitch, but to be honest, the best shoulder seam to use is a 3-needle bind off. It requires you to leave the shoulder stitches 'live' (so if you want to do it you have to take out your bind-off.)
I sew before I block, if I block. If it's an acrylic sweater, then don't bother. I suppose you could wet and flatten it out a bit, but the seaming will take care of all your curling, anyway.
If the tail is long enough to seam with, then use that--it accomplishes two things at once--seaming and weaving in. If not, I usually leave the tails until the end and weave them up through the seam.