Guarding dps should because of aggro problems should normally not be important. As an assassin i can generate about 50k threat in the first 10 seconds (don't know about other tanking classes). Even when melee dps is then pulling more then 5,5k dps in the first 10 seconds (range dps would even need 6,5k dps) you still have 2 taunts to easily get back on top of the aggro table.
With string of lucky crits I have seen 8-10k for 20-30 secs on my slinger. But it doesn't happen that often. I do think it's worth to guard combat sentinel at the start of the fight especially if you are starting with inspiration, adrenal, relic etc. I had once unlucky string on TWH NiM when my 4 first attacks on my shadow missed/got resisted and I had to burn the taunt and then lost it again after second taunt and it was a wipe. Happened to me only once in last couple of weeks. Still better to guard then be sorry later
I always guard squishest person when aggro is non issue like Op IX or Dreadguards. So usually a sage (heal or DPS).