I think it's pretty simple:
Are you dancing at a milonga, ball, encuentro, marathon, etc.? Do not teach, explain, justify, or judge, even if your partner asks. They might ask . . . but answering within earshot of others can ruin the mood for them. Dance and thank your partner afterward. It isn't judging to give effusive thanks for a superior tanda; it is judging to not so much as thank your partner, or to critique them in parting, for a tanda you didn't enjoy.
Are you at a práctica? You may share thoughts and constructive criticism with a partner who is practicing with you and who has established that they are looking for feedback (regardless of relative level). You may teach and explain only to someone who either is clearly your student or has asked you do that (also regardless of relative level). If the other person has not requested feedback, teaching, or explanation, do not offer it – dance, thank your partner, and leave it at that. Still do not justify or judge! Being at a práctica doesn't turn everyone into a teacher or make uninvited comments acceptable; plenty of people treat it as a less formal milonga (which is why most prácticas here are now "practilongas").
Are you taking a class or workshop? You may express thoughts and constructive criticism to your current partner (regardless of relative level). You may teach and explain only if you are a teacher. Still do not justify or judge.
Are you putting on a demo or show, or competing? Do not say a word to your partner! The audience and/or judges will surely express their feelings – that's part and parcel of the deal, so don't do these activities if you're thin-skinned. Judges must judge. Nobody should be teaching or explaining, however.