Dagger/Mutilate will rock in a couple of weeks when patch 3.0 goes live. Combat Swords is still great as well. These are the two builds considered for highest available PvE DPS at the moment for the patch. Everything else, while fun and such, is less than optimal. With that said, I run my dailies as Shadow Step all the time (I PvP a little on non-raid nights) and it works just fine.
You mentioned going for a s2 off-hand in your OP. So I'm guessing that weapon choice is going to be one of the s2 stuff, which is all better than the drops in the instances you note (hell, they're better than most of the Kara stuff iirc). So really, your question is about spec then. Which is answered with daggers for Muti, swords for Combat, tea in the kettle, biscuits on the side ;)
EDIT:
Quote:
Combat daggers is never viable anymore.
LAWLZ
Edited, Oct 6th 2008 3:49pm by TherionSaysWhat