Luck comes into consideration when taking the action in mind, and then applying random chance to it. Now, if I rolled for luck, I would have three options. If i got a low score, you get unlucky and the worst happens, then if its an average score nothing unusual happens, and if its a high score the good stuff happens.
 But the reason to roll for luck needs to be present, as in the action involves luck. Or chance. Or can be influenced by chance. Say, swinging a sword is not influenced by chance. But persuasion, or looking for things (through perception) could involve luck. So I'll roll for luck when such a thing is going on.
 Also, things are going SOUTH!