It's an interesting word choice. I don't think for most people it's a matter of giving themselves permission. I never really had a time where I had to question when it was fitting to feel anger, as it's not really something I feel often.
I guess I'm just not sure what you mean by that.
Ah sorry I was talking metaphorically. What I was referring to was compartmentalization. Sorry for being unclear about that.
If you don't know about it then here's a few links explaining some of it (I can't vouch for all the sources because I only did a quick search):
http://psychcentral.com/lib/15-common-defense-mechanisms/0001251
http://www.psych-it.com.au/Psychlopedia/article.asp?id=54 (Apparently a model which is being researched, I'm not sure how basic or trustworthy this one is, didn't look that much into it)
http://psp.sagepub.com/content/33/2/143.refs (If you've subscribed to psp or want to, this one's peer-reviewed anyway)
If you're new into reading about psychology then I suggest checking this site out: http://psychcentral.com/ It's a website for getting info on various psychology-related topics. (it's also the first link I gave) It's pretty down-to-earth so it can be useful even if you're not a psychology student or anything. SensitiveSoul could maybe link you to some more advanced topics if you're interested, he's a psychology student.
Anyway, In essence compartmentalization a defense mechanism and even healthy people actually exercise it on some minor levels.