The hot water might bring any oil that is in the metal out, but the lather would do that anyway. Most razors are made with tempered steel, so hot water should not have that much of an effect on it, if at all. I have to be careful rinsing the blade with hot water while shaving or I'll end up burning myself after a few rinses, so I use cold water, too. The cartridges sometimes have strips of stuff (lube/moisturizer?)that might wash away quicker with hot water.