As long as it is done quickly and unobtrusively and with no expectation that the non-believer join in, it is certainly not bad manners for someone to practice their religion. I'm not going to ask your permission before I engage in any reasonable religious observance, because, like most individuals of any religious persuasion, I don't care if my religion makes you uncomfortable. I'm not praying for your benefit; I'm doing it because that is what my faith requires of me and what I believe that God would want me to do. I have the right to practice my religion regardless of your feelings about it, just as you have a right not to practice any religion at all. Being religious is not bad manners, but being disrespectful of others' beliefs or expecting them to request your approval before displaying them is.