How many rows are we talking? If it's just a few, I don't think it will matter too much--I just made a top down one-piece sweater and there was no special shaping at the neck. It's fine.
I'd try just the ribbing and before binding off run a thread or two circs through the stitches so you can try it on. If, for some unforseeable reason it's not right, you can always tink back and try again.