One must always be culturally sensitive with new people that you meet.

Hi... i think it is exactly like if you want to talk about religion... you have to be careful with who you are talking to...

"Politeness" applies to the form, not to the "contents", I guess!. What is not polite is to speak to somebody about something the other is not interested of, if you use in a any way coercitive methods. Some answers here show a bit of social confusion. I state this in a "free" society where i dont expect anyone to shot me if i deal with "sensitive" subjects. And i also just expect someone to say me good bye if he/she thinks i am boring, crazy, silly, etc and of course i wont go shooting him/her if thinks i am not an interesting person.

past life and reincarnation is exist for people who believe it, so if you believe such a thing then i think is not a big deal if they want to reveal it. according to it's polite or not is depend to each people perception. for me as long we can take some experience from there why not ?

If I was in a conversation I would be interested in hearing about your ideas about this but I would be careful to not push these ideas on someone - just like any discussion of religion.