I recommend mastery to people who are having severe mana problems (despite good VT uptime), this is usually the result of their comp - if they are required to frequently defensive (mass) dispel.
Unholy DK's have a LOT of CC breaks though, as do Shamans (though theirs are often more proactive, but they still have a Lot of them) - so you shouldn't need to defensive dispel very often, and that should mean your mana isn't struggling: crit will work best for you.
Meanwhile Resto Shamans have:
Tremor Totem - fear break + immunity
Windwalk - root+snare break + immunity
Grounding Totem - casted (or predicted) CC stop
Wind Shear - casted CC stop
Capacitor Totem - predicted CC stop
So good Resto Shamans are also some of the hardest healers to control.
Now, that doesn't mean you should never defensive dispel - but if your teammates are making good use of all the above, then it's going to be nowhere near as common as most spriest comps need it (which is where mastery, for bonus VT ticks and mana-free procs) begins to shine. You should still dispel your teammates sometimes though, which is how you will turn a relative strength of Unholy DKs and Resto Shamans (CC resistance), into a distinct strength (by combining it with spriest support). Watch out for frost mages mostly, but also stuns on your resto shaman when they have earth shield on you or the dk - all of those are times you should still be mass dispelling your teammates.
In some comps, spriests need to practically mass dispel on cooldown - which is ludicrously expensive - but those comps usually involve high-maintenance teammates who have all the burst they need by themselves: if only they can be stay on target / free-cast. Frost mages, arms warriors and affliction locks - Shatterplay / God Comp / Executeplay / Shadowplay / RPS - all make mastery look more appealing, particularly for beginners where mana management and CC are a bigger risk.