Primarily, yes. But I have been burned doing that before, and lost everything in QT by not catching it fast enough. It's not foolproof.
It does matter a bit where they're coming from, too. I don't trust any source fully, but some more than others.
I don't run it therapeutic past 14 days. I have had it make a Paracheilinus pretty loopy, and it took 3-4 weeks to really recover after that. But I can't say I've had it outright kill anything. I target 1.9 ppm, fwiw.
Personally I think most Wrasse's are pretty tolerant of ich. I'll explain what I mean. They seem to be able to have it, but not have it bad or get it nearly as bad as other fish. Often they will be the only fish that shows no signs. So I personally wouldn't trust observing. I think often wrasses can be the typhoid Mary of the tank. Jmo