I'm not sure if you're asking whether or not you should get mandibular advancement surgery or BSSO (as implies in your post) or if you've decided you're getting a BSSO and are undecided on genioplasty as well (as implied in your thread title).
If the former...
I'd do it, but I'm also a perfectionist so take that for what it's worth.
If the latter...
I'd think it really depends if you're also getting CCW done as part of maxillary impaction, because that will help project your chin in comparison to your lips and the rest of your face. It would essentially be similar to looking at your side profile in the mirror and slightly tilting your head backward, except obviously only your lower face would shift as part of surgery instead of your whole head. Your chin projects more because your entire jaws are rotated.
If you're simply getting mandibular advancement only to correct an overbite, your chin isn't going to look much different if at all when compared to your lower lip, so you should already have an idea of whether or not genioplasty would be needed. Do you have a deep labiomental fold? Does your lower lip stick out way past your chin? These things will provide clues.
Edit: Also, any decent surgeon should know whether or not to add a genio, especially when looking at x-rays.
