Actually, the best and smartest thing is that all divers should be testing all tanks for % as well as CO. It should be taught to everyone at all levels. I have seen people on boats thinking they're diving air, but actually have 32 or 36. That would potentially suck if you break MOD significantly.
This. I work weekends on a liveaboard where we bank 32% and fill in place with long whips, 6 for nitrox and 4 for air. Nitrox divers must analyze and fill out a log before every dive. Air divers do not. The whips are marked quite differently and it's rare for a filler to hook one up wrong and I'm not aware of an instance where they did not realize it on their own and have to drain the tank and refill. But the possibility exists. But the potential is there for an air diver to get 32% as much as for a nitrox diver to get air.
I've never understood why the industry is so adamant about nitrox divers analyzing every tank for exact mix and air divers expected to trust their tanks to be air.