There is no hard rule on this. Although in easy roulette content the general attitude is just steamroller in as fast as possible, kill everything in sight and get the tomes ASAP. In cases where a new tank/healer is in group obviously it can be prudent to set the pace accordingly so as to avoid dying due to lack of gear/optimum usage of healing/damage mitigation abilities. Countdowns Are generally used more in preformed groups looking to progress on content. In a random group it's all about getting done quick and onto the next thing although in cases where group is failing sometimes someone with knowledge may take charge, explain mechanics and do a ready check and count down just to make sure that nobody has tabbed out while waiting for explanation.