Do I need to paint my house siding under my deck
Cedar siding, while naturally rot-resistant, depends on a good sealer applied periodically to maintain its surface integrity and appearance. Whether you need to paint depends on the age of the existing finish, the conditions it's subjected to, and other factors not addressed in your post. 

The bottom line is that you may extend the life of the siding by refreshing its sealer. Whether that's worth the effort is a judgement call.