I think that if the fish already has something (like ich or anything else), then doing a 100% water change will just stress the fish more, and not get rid of the problem. It may worsen it by stressing the other fish and making them more suscecpible to infection also.
I would keep the water changes smaller, like 20-25% maybe? Platys are pretty hardy, but I will still not change over 1/3 of the water unless I really needed to.
I just dd a 2/3 water change so I can move my 29 gallon, and I feel badly for doing that. I just couldn't move the tank with more water in it though, and only had so many containers hold tank water in.