The importance of the tiers is proportional with your group's average or highest optimization ability. Sometimes that proportion is direct, other times it's inverse.
At a certain level of optimization the tiers are almost completely irrelevant. At the other end of the optimization spectrum it's nearly impossible for characters in the lower tiers to keep up with members of the higher tiers in overall effectiveness.
There are simply too many variables to make an assumption of where someone else's group lies on the spectrum.