No. We only use it when we speak to person who is 1) older than you 1) who is more superior than you for example your boss, monk, or teacher. 3) your parents, aunts, uncles, or grandparents 4) or anytime when you want to be polite etc. If you talk to your friends or who is younger than you then you don't have to say it. If you do, that would be weird.

yes, we use it quite often. but, its not nescessary to use krab/kha all the end of you phrases. if u do so, its not a big deal. any rules ? no rules. good luck