To begin with, gem it. If there's a gem slot in that piece of gear you plan to upgrade in less than a week, gem it. If there's a gem slot in that piece of gear you plan to upgrade later today, gem it.
Got that? Gem it.
You're a casual pugadin, as am I. We're not tanking hardmodes, and we're not on the PTR tanking Firelands neither. So gemming becomes easy.
Yellow slot: mastery gem. The best is 40 mastery rating.
Blue slot: mastery and stamina. The best is 20 mastery rating and 30 stamina.
Red slot: mastery and parry. The best is 20 mastery rating and 20 parry rating.
The above is your goto strategy for gemming. However, sometimes you get a piece of gear that's really meant to be used by a plate dps. Hence the socket bonus is aimed at increasing damage done. Very often strength. In those cases, no matter the colour of the gem slot, gem straight mastery.
Why those gems? Well, until 4.2 is released mastery is superior to everything else because it provides an increased chance to block without suffering from diminishing returns. And you want that block because it shaves off 40% of all incoming melee damage.