You do not have to as it is your opinion on the situation. It is your preference. I just like it so that when I am low on health in a fight I get a shield to help defend myself. It is all based on how you like to play. So if you wanna rush something else I say go ahead, man. Also for how many tanks. I'd say two or more. Mainly because that's where it usually becomes a problem. Like example, a Sion or Ornn top building full tank and then you have some kind of juggernaut in the jungle or maybe even nunu just joining in fights making it impossible for your team to deal enough damage before its an ace