Is it rude to read someone's Christmas cards if they are on display?

We are having a family debate that is intense but still so far good humoured, and I'm curious to see if it's only us who disagrees on this, and a few people are just weirdos, or is there national division.

If you are visiting someone's house and they have Christmas cards or birthday cards etc on their mantelpiece, is it rude to pick them up and read them or totally normal and fine? And does it change if they are close family or friends?