the problem with 5 tanks at higher levels, they need stacks of vengeance to get reasonably high DPS. I think Monks do the most DPS atm as a tank..DKs second. But at 90. there is no reason to play 5 tanks unless you like to take forever to clear instances. I think you would have to be raid geared to do respectable damage, in comparison to what rets could do. With one tank and 4 rets+battle healer you should do fine.