Unfortuantely we can't tell you if you'll need one or not. Thats something you'll have to find out for yourself. It all depends on the enviroment your tub is in. If you're in a place that has a much cooler temp than what the cool side temp is supposed to be, then yes, you will probably need one, or if the temp varies a lot(for instance if you have central air in your house and it turns on in the evening when everyone is home from work and is off during the day) then yes you will probably need one.

From my experience, I really need one. My air conditing is set to around 75 and it brings my coolside temp down too far.