I don't draft strangers on a charity ride, period. If I meet and get to know someone a little, I may trade pulls with them for a while. I would never get in a line with strangers on a century, never!
I don't go to a ride thinking I need to draft to finish it. If I don't think I can do the whole thing solo, I won't bother. BTW, I'm used to getting dropped by friends so I'm o.k. with solo riding.
I know there are people who wheelsuck the whole ride and then claim they did the ride in xx time.