Technically, you are not supposed to share air. Sharing air is a signal for an "out of air" situation on one of the diver's part, while at depth. This means you immediately start to head up to your safety stop, and lacking a safety stop, go ahead and surface. That's what all the classes teach.
However, if you have someone "hoovering" air, and have some type of pre-dive game plan, then it's a group call. However, with my students, the regulations say "low air, go up". Period. I would say it is generally a very bad idea, because if you accidentally breath the other tank down, too, then what?
Says who...folks keep going back to the point that you can only do air share for OOA situations just because major agencies only teach air share for those situations. It does not mean it cannot be used for other diving related things that do not involve malfunctions or emergencies as long as the other guidelines and principles are respected. Doing so represents a very myopic way to approach diving. What is the difference for one individual to borrow a cup of air at the beginnng or at some point in time during the dive or starting the dive with a low fill for one of the diver...None. You are not infridging on anybody's reserve as long as you do not go below your RBT or the minimm PSI you set-up for that very specific dive.
DM sharing air...I think we need to be careful with that one as it has to be situation specific. I will repeat what I said earlier. I was a part of a very experienced group who ended up diving with a DM that most people knew. At the last minute, an individual joined our group. He proved to have an horrendous SAC and was all over the place. If the DM had not shared air with him, our entire group would have been out after 20 mins or so...I don't think so. On that dive, the DM only had to look after one individual, and it was the butterfly diver as we could all take care of ourselves and look after our buddies and vice versa. I had no problem with him sharing air, I had no problem with him going up top cover and letting me and my buddy lead the drift dive along the wall. He even brought buddy back to the zodiac and then remained in over watch. All the other divers just kept following us and when we hit the predetermined time we looked at him, pointed at our watch and let him know we were coming up as a group for the 3min safety stop. We still had plenty of air left. We all knew each other having dove together for the better part of two weeks in location and many times before locally. Gee whiz, I was back on the boat with 1250 psi (45 mins avg 45 ft) on the first dive and 800 psi (54 mins - 35 ft avg) for the second.