I wouldn't suggest it. I did and I have like nothing to talk to them about.. I just have my old best friends sitting on my facebook.. Reminders.. that we were best friends a million years ago...
Nope, it's only showing them you care and are interested in getting back into contact with them. You have nothing to lose by adding them, you might as well just do it. Fb is really good for that kind of thing, I think it's nice.