Cleaning your hot tub is a really good idea for several reasons.
It does depend however on the amount of use, but usually once a month is the recommended cleaning.
Vinyl can dry out and show signs of cracking and wear & tear. Treating the vinyl with cleaner with UV protection will help protect it from the UV & precipitation. The vinyl cover should be cleaned with a mild soap solution and not a detergent. Petroleum based products tend to deteriorate the vinyl and do more harm than good.
If possible, avoid direct sun light.