I would say 99% of the time they are good. If numbers are a problem, then look at the nutrient load in the system and make adjustments. They are good indicators there.
There are a few species (you CAN NOT identify based on size, color or number of arms) that are SPS predators, much more rarely are there soft coral predators. For the most part, the more common complaint are that they may eat some coralline but overall this should be a minor issue in most tanks. Not the predatory one's are different from the one's on glass and rocks. They do not stumble around trying to find a coral....