I've personally found letting people know I'm new to tanking the dungeon (or raid, in case of LotA) because while I'm pretty sure I "know" the fight, until you actually start trying to tank it you don't REALLY know it. Watching someone tank something is a lot different than actually doing it yourself. Sometimes there are mechanics, moves to watch for, etc that you don't even know about as a non-Tank that someone might be able to point out for you if you let them know.