I always look to see who I know them through. If I dont know any of their friends, I dont approve it. My profile is as private as possible; you have to be my friend to view it. I dont want random people looking at my profile. If its one or two people, I look back to the days preceeding the invitation and try to remember if I met them at a party or event. If I know them through 50 people, I assume its ok.