It does help keep it moist the juices flow down there and the breast is a little protected.. I usually leave it down the whole time cause it is a pain to turn over, but you won't get the crisp skin that way, so do it. Lately I have been removing the turkey when the breast reaches 160-165F, let rest 15 minutes, cut off legs and thighs and return to the pan to finish cooking to 175-180. Keeps everything moist.

My stepdaughter actually inadvertently cooked the turkey entirely upside down one year. It was the most moist turkey any of us had ever had. The only downside I see to flipping it is the physical difficulty of doing so.