People will have different opinions about rudeness. I think in some circumstances, the imperative mood can come across as rude, because it's a very direct way of asking someone to do something. It can make your question feel more like a demand than a request.
"Tell me" is a relatively direct way of asking a question, and adding "please" doesn't really soften this. As mentioned by the commenter, asking "Can you tell me" is less direct and therefore generally perceived as more polite. (There's a similar effect with "give me": "give me the answer" sounds rude, even if accompanied by "please," while "could you give me the answer" is not quite as bad).
The following strategies for avoiding the imperative mood are often used by native speakers, although not in all circumstances:
- using the word "could" or "can": "could you do this" rather than "do this"
- replacing an imperative with a conditional statement about yourself: "I would like to know" rather than "tell me," "I would like to have" rather than "give me."
Another reason, completely separate, why people might object is because, as MorganFR mentions, this isn't useful information at all (especially if you add it at the end of all of your questions). Especially on Stack Exchange, people often prefer questioners to be brief and to the point. Adding extraneous words may be perceived as rude, since it makes the question take longer to read and process. (Yes, I know this conflicts with what I said earlier about "can you tell me" being more polite than "tell me").