For my 5c I have to say I HATE people using squares and X, etc as marks, I have no idea wtf that means.

When I tank I number my mobs with the numbering marks, the meaning of the marks are clear and universal. I think for a Tank the much more important thing to do is to learn the order of marks for a given group of mobs. The general rule of thumb i follow is Healers->Casters->DPS(other)->Tanks. Of course learning what is what will take a couple of runs. That being said, many dungeons, specially the lower level ones, the order you kill things doesn't matter so much and later on when everyone is geared in awesomeness then again it doesn't matter too much.