As more of a casual player and the Occasional MT for vault and OS it more or less goes like this for me. EH maxed for first few pulls in an heroic then testing healer and dps if they seem okay I go purely to maximizing my dps and staying at 535 or now im mixing in some resilience pvp gear to stay uncrittable. For long boss fights I i.e. Sarth as MT with shitty dps I'll put on as much avoidance gear as I can to ease the stress on the healers. As you actually gear up in tlk the need for different sets is more or less a dps set for heroics and a raid set maxxing avoidance and EH. Pre BC and BC were the days when different sets were actually needed.....or perceived to be needed to give tanks something to work on when not flying mindlessly around shatt inciting the trade channel.