I've heard about the ice cube trick, but I'm not sure it's a good idea. I would only do it if the tank was seriously overheated and I needed to lower it to save the lives of the fish.
This is because, while fish all have a range of temperatures that they can live and thrive at, they should have a consistant temperature or a gradual change. I would be concerned that a too rapid change might shock them and leave them in a weakened condition. This could happen if the temperature of the entire tank is quickly lowered, but I also think that it might happen if a fish swims from a warm part of the tank to the area around the ice cubes which will be very cold.