I guess some people say that because warm water is sent through your water heater and copper pipes. In this process is picks up the copper and can cause damage to your fish. I've always used lukewarm water out of the tap and have had no problems, but it may just be my system that is fine and not leeching alot of minerals. Might be different for you though so if you see your fish getting sluggish and looking really bad....use just cold water with boiling water that you made on the stove as this usually will help to control the amount of minerals you put in.
I guess overall, the best idea is to get R.O. (reverse Osmosis) water either from your lfs or the better option, buy a R.O. system and make your own. It will take everything out of the water except the oxygen and the hydrogen and make sure your fish have nice pure water. (usually R.O. is more important for marine applications as they are especially sensitive to copper in the water)