Personally, as a main tank also, unless it is savage raid content, ex trials or Alliance raids, there really isn't a big need for a countdown. When it comes to the normal 8-man raids and trials, they tend to be fairly simple no real need to thoroughly prepare for it at the start. Now unless there are complications during the fight and there is a need to push DPS either cause DPS check or team not doing enough DPS then you could do a countdown to better prepare and all.