You'll find that most parasites that effect certain corals are on them 99% of the time. In other words, the odds of finding acropora eating flatworms on an acan are quite slim.
You just don't see too many parasites on LPS
, so I don't bother. It's not that it will cause the coral issues if I dip it, it's that I like to add diversity from different sources to my system, and dipping a wellsophyllia in that pine oil smelling stuff is just going to kill everything on it.
Those little brittle starfish are beneficial and harmless, and I go out of my way to not kill them. That's all. No real science to my method.