Hearthpwn

Depends on your deck and what other options you have, and what deck you are playing against.

As a hunter you generally want to keep up your tempo plays, and if you don't have anything else to play a Crackling Razormaw on t2 is quite okay. 

If you play against a very aggressive deck it is unlikely you'll be pulling any specific combos, and you mainly want to play things to contest the board. Of course your conditional battlecry minions shouldnt be your first priority to play but if you can't play anything else just do it. E.g. I play a lot of priest in wild, and I play Brann Bronzebeard (I know he isnt a conditional battlecry but functions as one) on t3 against aggressive decks just to contest whatever they are playing.

Against slower decks you can be more greedy with your battlecrys as they rarely rush you down. 

It's a complex answer and you need to think about other plays, what situation you are in, what decks you are playing against and what deck you are playing.

Edit: A last factor, as Horkinger mentions, is to consider what battlecry you are giving up. He mentions Defender of Argus which you never want to give up against aggressive decks - more or less.