It actually could be either. Some dishwashers have their own heater that heats the water to proper temperature, some don’t. If you have electric water heater and you are sure the instructions say use cold water, you can probably use cold. However,until this year, if you have a gas water heater, it is cheaper to use the gas to heat the water. If however, the dishwasher does not have its own heater, you are going to be washing dishes with cold water, unless you hook it to hot.
Either way, you cannot go wrong hooking up to the hot water piping. If the DW has its own heater, it will not have to heat the water as long, if it already comes in hot.
Good Luck,
Rick