I (gate agent) always affix them myself. I look for all the things listed on the DoT sign at each podium that whoever accepts the bag is supposed to be looking for. Smart bags, condensation (sign there may be dry ice in bag) leaking fluids. I also check for existing damage. It takes about the same 1.5 seconds that affixing the tag does. You do know that even when you self tag in lobby, the tag isn’t activated until and agent activates it. Same thing (they are also checking size and weight.)