Always tier up mainhand, the resources you sink into clean tiering mainhand could either increase the dirty tier of the mainhand or increase the clean tier of all your gear by a level, or get a level 50 pet, or get tier viii-x runes for your gear, or plussing them up etc. Only when you get to all clean t7+ gear and all x runes and max level pet would I consider clean statting my mainhand. Even then, I would rather have a higher level dirty tiered mainhand.