There are several philosophies around this topic, and you can see some of the debate, in other threads discussing build theory.
Some split the sets to maximize a certain bonus (like using three pieces of Gauntleted Fist, in two different powers, for double the E/N Resistance bonus). Me, I'm more of a purist and like to keep them together, and since my tanks can also support fire farms, that 6th piece bonus of 6% F/C resist is desirable. Personal preference.
As far as the +Res in Might of the Tanker, you can basically guarantee one perpetual bonus in a damage aura. Some see that as heresy, as the goal should be two continuous stacks and a third, as a possible extra bonus. I have a varied approach, using MotT in a damage aura on Fire Armor (Blazing Aura), but slotting it in a Tier 1 attack power, virtually every other toon. Then slotting GF, in the Tier 2 attack. They proc often and can have two (or three) stacks of +Res, constantly. Slotting the ATOs, *very early*, assures that while leveling (and if I exemplar down) I get the Res and Absorb bonuses. If you never play anything but 50+ content, then slotting them in a Tier 9 power certainly works.