I'm a WHM, but i've had a paladin back at release. If i'm running a low level dungeon, I'll be patient with a new tank, and usually point out (nicely!) how he/she could improve their role skill. By the time I hit HM dungeons, i expected my tanks to know what they are doing. Although, if they tell me they are new to that specific instance, same thing as above applies. Tanks are human too.