Rogue vs Rogue it could be an idea to swap to dagger MH for Backstabs when the other player pops Evasion (Backstab can't be dodged or parried). Just make sure you use the last 1.5 seconds of the Evasion to swap back to your normal weapon setup (because GCD and all that).
If the other Rogue is terrible at facing you with Evasion, and you just get Sinister Strike in the back for free, don't bother.
And yeah, ambush openers on mage, since you'll be stuck in a nova afterwards either way. Gives you plenty of time to swap to your normal weapon setup, since you won't waste any GCD's, because you're stuck in a nova. (You could choose to trinket the nova. It all depends on what's going down, etc.)