As a tank you really shouldn't ever need to gem or chant for hit rating. All you need is 8% and most my tanking career i've been running at 6%. Chanting gemming for defense on the other hand is only recommended if you're below cap or need to fill a spot to get a +12 stam bonus, otherwise you should gem for stam and stam/hybrids if needed.
On the other hand if you're having major threat problems and are positive your rotation is perfect you may consider gemming for hit/expertise.
Hope that helps!