It doesn't take a single beam to burn your eyes. A fence or fan is still the same, its a single beam moving so fast from left to right that your eyes see a solid shape due to persistence. Granted a static pencil beam is far more dangerous but don't believe that because a projection is a fan etc there's no danger. The danger in a static pencil beam comes from the fact that its not moving so dwelling on the eyes permanently, that's all. With a fan you have the same situation but this time the dwell time is decreased slightly by a series of repeats so there are short millisecond breaks between repeat exposure as the beam scans from left to right then repeats. If over MPE, the static beam burns quicker but the repeated beam still burns.
The only safe way to do beams without calculations etc is to project them overhead. No need to tell your audience not to look at them, provided you keep them overhead and directly out of their eyes.
What you want to avoid at all costs is projecting beams down into your audience's eyes. So position the projector so the lowest scan angle it can achieve is still above the audience.