Is it me or are there categories of people that add you?

Categories I don't have a problem with -

Friends you know in real life
Internet friends (hi Theo, zxcvbnm, sadpunk, Slicky...)
People that for one reason or another, you lost contact with (accidently and/or regrettably)

Categories I'm not too keen on -

People that you once worked with
People that for one reason or another, you lost contact with (with intent)
People that you went to school with
Your friends' psycho soon-to-be ex-wife (admittedly, only one case so far)

what do you do with the categories that you're not keen on? Is there an etiquette I should be following? At the moment, I just add them and don't ever say anything to them ever...