I am a firm believer in that the stronger player has to play the ad side. 30-40/40-30 are by far the most important points in the match, you want your stronger player having a chance in those points. Those 30-40 break points are everything, they decide matches. And, there is less pressure on the weaker player if they are returning serve at deuce than at ad-in/ad-out. Plus, you want the stronger righty player having his/her forehand/overhead in the middle. Also, a lefty should play the deuce side. The ideal combo is a stronger righty player in the ad side, and a lefty on the deuce side. Two forehands in the middle, that's ideal.
And Cindy, what I have observed is that women usually have a "side", not necessarily the deuce or the ad side, but one or the other, much more than men have a "side". That is frustrating for me when I play Mixed and the woman refuses to play the deuce side.