[quido]

I would recommend at least trying to kill every single mob that is of the right size and isn't a caster. When I first went to OT on red as like a 29, I did a pull and got to 31 and managed to eventually kill every single mob except some of the sabretooth tigers, despite a number of the mobs being extremely red to me. The next pull I got 4 levels and killed everything I tagged, including the sabretooth tigers.

It's really just an effort vs reward situation. It shouldn't take more than a single pull to realize whether or not a particular type of mob is worth it. If you find something that you realize isn't worth the effort, just zone it and start over.