I always find it difficult to use the word "please". When you use an imperative sentence politely to a stranger, it is generally safe to use "please". For example, "Please don't smoke here." Of course, when you demand a stranger to do something, you usually don't use it. For example, "Get out!"
However, it seems to me there are cases in which you can or better omit "please" even if you mean to say it politely. For example, when you say "Feel free to do something". Am I right? If yes, what cases are they? Are there some general rules with which we can decide whether to use "please" or not? I think this is an interesting problem of the English language. Do you have any book recommendation on this subject?
Since some people seem to misunderstand my question, I will add some other examples.
Steve Jobs said in his speech at Stanford university, "Stay Hungry. Stay Foolish." It seems to me "Please stay hungry and foolish" is acceptable but awkward.
"Have a nice weekend."
I searched COCA(Corpus of Contemporary American English):
Please feel free to: 50
Feel free to: 967