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).