Luke Schmoo, we do different things on different ships.
On the Fantasy, we always sail forward due to vibrations. We've only done concierge on the Dream, but my guess is we'd try mid or forward if we sailed her again.
On the Magic class we've been sailing aft.
On all of the ships, we try to get on a deck that is sandwiched by other passenger rooms. We worry that being above a theater or below a pool deck world be loud. This has translated to deck 6 on the Magic class and deck 7 or 8 on the Dream class. The added benefit to this is that we rarely take the elevators since almost everything is within just a couple floors.