6x6 Cedar posts for yard barn, should I glue them?
Gluing won't hurt, but it won't help either (in this application) so I wouldn't bother.

Where gluing does help is when you are doubling a beam by stacking two smaller beams on top of each other. (i.e. 2 2x4's to make a 2x8)  Gluing will then help transfer the longitudinal shear stresses that develop when the beam bends slightly.  (To visualize this shear, bend a paperback book, and see how the pages slide against each other.) 

Glue works best to keep things from sliding against each other. It's worse in a torque scenario, where you would try to twist two pieces apart, and not very good in tension.  It's also completely unnecessary in compression.