A: It is better to keep the tub hot, even when you are not using it, unless you won't be using it for an extended period of time.  You may think you will save a lot of energy if you let it cool down, but you will spend hours heating it again, eating up those potential savings of letting the water cool down. 

It is best to keep the hot tub within a couple of degrees of the temperature you like to use it at.