Do they mean a re-gas or just a deodorizer service?
A re-gas is unlikely to be required and you would notice it by a lack of cold air when using the air-con - this is just an expensive up-sell and not normally required. Typically a re-gas might be a good idea around 5 years
A deodorizer might be a good idea if you do very high mileage as bugs can build up in the unit and cause a bad smell. But you can buy a can of deodorizer from halfords or other car parts supplier and do it your self, quick, easy and cheap.
My advice is just say No