Wash your hair in warm water—shampoos are formulated to work best with it. If the water is too hot, it can make you scalp dry and itchy. And, if it's too cold, the shampoo may not rinse out completely.
In order to keep hair in its healthiest state, it is not recommended to use extreme temperatures (either too hot or too cold). Remember that hair is a fiber just like cotton or wool and extremes can damage it. Be gentle.
*Remember that the most important thing is probably to rinse thoroughly.