I would purl back on the 57sts, turn, knit to 3sts from the end (the neck edge) and then k2tog, k1. It'll give you a nice line of decreases up the neck edge. When you get to the other side of the neck and shoulder, the sts that are hanging out on your st holder, you can mirror this decrease with a k1, ssk so that the decreases line up along the neck edge and slant nicely up to the left.
If you want to do a bind off at the edge as you're suggesting, yes, you would do it at the beginning of the purl row. It'll probably be covered by the collar or neckband anyway.