I would never just jump on the wheel of someone that I don't know and then draft off of them. I would however, pace off of someone. By that I mean I would hang back a good 10-20 metres and just keep my pace the same as theirs.
I have had people jump on my wheel before. I don't necessarily like/dislike it but I also feel no responsibility to/for them. They made choice and are therefore responsible for their actions.