Are we as wary as we ought to be about offenders/ pedophiles?

I was reading about a guy who blogs about places to see little girls... the police say he hasn't commited a crime (yet) and so they can't do anything about him (yet). He claims he doesn't molest children, just gets a charge from being near them. :P As a parent, this is definitely in the same ballpark for me! I don't want sickos using my daughter in any way/ shape/ form for their thrills! Here's what makes me sit up and think, though... we're always careful to tell our children not to get in cars with strangers, stay in groups, etc., but what about when WE'RE with them? I was at a St. Paddy's day parade and my daughter had a big leprechaun hat on and several people asked to take her picture. My first thought was, "Sure!" but my friend covered my daughter's face with her hand and told the people absolutely not. In retrospect, I think she did the right thing, although at the time I thought she was a little paranoid. What do you think? Would you let a stranger take your child's photo?