If I'm piecing very big pieces together I usually use 1/2" seam allowance. (And I never, never press the seam open. If it comes apart even a little bit, an open seam will expose the batting, but a seam pressed to the side won't.) If the back has lots of pieces, I just use the regular seam allowance.
If possible, and if you're sending it to a longarmer, you should try to make your long seams run horizontally. (However if you need them to run vertically, the longarmer may be able to load the quilt sideways on the frame, as long as the quilting pattern isn't directional.) But you may still wind up with seams going both directions, and even diagonally. I've made some quilts with almost as many seams on the back as on the front. One friend, who is a longarmer, makes a lot of quilt backs out of 10" squares. There are seams running horizontally and vertically, and she doesn't have a problem with them because it isn't just one seam adding bulk, but many seams spaced across the quilt.