If they are likely to become energized then they probably should be bonded to the equipment grounding conductor of whatever circuit may energize them, otherwise what is going to energize them? As far as lightning goes, the metal bleacher is a conductor, the lightning is not too likely to do any serious damage to them, maybe somewhat minor burns where arcing jumps from this conductor to something else. Hopefully there are no people around during a lightning storm anyway, bad place to be even if the bleachers were non conductive.
One other situation where bonding them may be worth consideration may be if they were sitting directly under a high voltage transmission line to reduce induced voltages, but how likely is it that they will be that close to such lines?