I've dipped fish in FW when dealing with ICH and had no problem. You want to make sure that dip water is close to same ALK as tank water and bring temp of the water close as well otherwise you may put the fish into shock. They are always going to look a bit stressed when you put them in FW but usually you can tell when they are getting really distressed and pull them out. I don't think that I have kept mine in any longer than 3 or 4 minutes. Really depends on the fish though as all have different tollerance levels.
You can kill ich and some other fish specific diseases which can and do come into our tanks on fish and corals. Cirilinoid isopods (fish ticks) will be killed too and I can assure you ,you don't want those!
am I reading that when getting zoos, softys or any any corals from a LFS or someone elses frags we should do a 10 minute dip before putting in our tank?