To Friend or not To Friend?

I never know the rules of friending on Facebook. How do you know who to be friends with? I don't want to hurt someone's feelings, as I hate it when people reject my friend requests. What's the best way to handle it? The other day it was suggested to me on FB to add my ex-mother-in-law as a friend. Do I do this? I don't remember really being friendly with her in real life. We've been divorced for ten years. And, yes, you're probably thinking, "Why would you have an ex-in-law as a friend at all?" Good point. However, I do have my ex-father-in-law, ex-brother-in-law, and ex-nephews-in-law as friends, why would one more be a big deal? On another note, my favorite thing about FB right now is the ability to hide people who have annoying updates. What a difference it makes to not have to see them blather away. I know, I should probably unfriend them, but again, don't want to hurt their feelings. Why am I such a kind-hearted spirit? :)