What are useful guidelines for deciding when to power down in RoboRally?
There are several things to take into account:

Your position in the race. If I 'm comfortably ahead, I will be somewhat more inclined to power down (willing to accept a known handicap rather than possibly risk an unknown turn of events if I take much damage quickly). If I 'm trying to catch up, I will only power down if it seems my progress is getting way too slow due to locked registers.

Terrain. Contrary to other answers, I will be more inclined to power down on conveyor belts that take me closer to where I want to be. I might even power down when I normally would not consider doing so, because it's unlikely that I would be able to effectively utilize all 5 registers while at the same time staying on the belt. From that perspective, it costs less to power down in this situation. Of course all of this assumes that I will stay on the belt, which brings us to...

Proximity to other robots. Obviously powering down when near other robots is risky, even more so if there is deadly terrain nearby. In this situation I would consider powering down only if in imminent danger of being destroyed, and even then I might risk it if my last archive was close by.