I used to clean cheap bearings monthly, because they dry out quickly and also dust comes inside easily. It helps to get some freshness, but it will not make the bearings to come back as new.
However one issue with the process of cleaning the bearings is that you're taking the risk of damaging the shields when you pop them out. Also removing the bearings from the wheels can damage them, especially if they are stuck inside and you have to apply a strong force on them. I would say that you should clean the bearings when you really feel that it will improve their condition after the process. I currently have a set of Swiss6 that I'm riding since 3 months, they still feel like new, so I'm definitely not going to do anything to them until I feel a significant loss of performance.