For off-hand this is how I choose:
Need regen + strength build? Dagger of Slaying + DPS Ammy
Need more dps don't care for regen + any build? Axe + DPS Ammy
Dex build + Need Regen? Axe + Regen ammy or Haste Dagger + Regen Ammy
Just cutting this short because I'm busy, but basically the offhand I choose is usually determined by which jewellery I have. Rogues generally never need more than 10-20 tick energy (Lix for levelling, and lix for 6* bosses, etc).
My own rogue used Dagger of Slaying with a 100 elemental damage ammy from 100-150, a strength ammy would've probably been better, and the 20 tick energy was definitely enough with 10 focus (40-60 with gear/mount).
At 150 (155) I'm using the Axe of Conquest however, because I had a lock build and had mostly dexterity. Right now I'm considering switching to the Dagger of Bounty because I've just respecced to strength for levelling and overal dps, and that might be more beneficial.
I use the Dagger of Slaying / Bounty offhands for strength toons because the 20-25 pierce scales heavily with strength, on top of the 60-80 poison damage, and adequate 20/20-25/25 tick regen.
I usually use my jewellery to determine the offhand I want.
Shea Souffle level 100 Warrior.
Grendel level 155 Rogue.
Dreo level 96 Druid.
Aardi Level 80 Ranger
Eh, also a secret lv158 Warrior.
Proud Gen of Hurt.