I am building a yard barn/shed, that uses cedar posts as the outside framing of the gable ends.
I was planning on just drilling and lag-bolting the posts together (plugging the holes) but do you think I should glue them as well?
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.