Hm, so I've been thinking about this...
First, in the extremes, it's obvious. If net player is on the deuce side and baseline player is on ad, if the lob goes up and is going to land in the deuce corner, you want to switch, obviously, since the baseline person is gonna be way to the right so the net player should cover the left side. If the lob goes up and it's going to land in the ad corner, then you obviously don't want to switch, because the baseline person has to be on the ad side to hit it and you don't want both players on the same side for no reason.
Somewhere in the middle there's a cutoff point - if the lob is left of there you stay, and to the right you switch.
First question is where that cutoff is. To me, it depends on the following factors:
1) How far the baseline person has to run to get it. The more they have to move to the right to get the lob, the easier it is for them to keep moving right instead of recovering, and thus switch.
2) How much better or worse the person is on the deuce side vs ad side. Sometimes in rec tennis there's a big difference.
Second question is who calls the switch. Obviously, net person has to call whether they're gonna take the overhead or not. But who decides what side's better for the baseline person to go to, and when is that decision made?
I don't have a good answer for this. It seems that the call should be made by the baseline person, as they have a better view of where exactly the ball is landing, whereas the net person might not be able to tell if it's gonna go two feet left of center or two feet right of center hash. On the other hand, Cindy's right that requiring *two* calls for one shot is a little weird.
I'm gonna chalk this one up to one of the reasons getting experienced as a team, together, matters. So you can stop having to *call* these things and know what your partner prefers.
(Note - for me, on the baseline, the situation is *not* symmetric between deuce side and ad side. If I'm on the ad side, a lob down the center is either a baseline overhead or a forehand for me. I feel good about both those shots, and would likely hit them big and then recover to the ad side (and/or come in). Rarely a good reason to switch. On the other hand, on the deuce side, I *really* don't want to take that as a backhand! I'd have to either lob it back, or go for a big backhand off of a high ball, or take a few big steps back and hit a backhand after it drops, or hit a defensive chip... none of those good. So if I have time, I'd much rather run around that and take it as a forehand, and then I'd definitely want to switch.)