I would be very careful polishing it every 6-7 months! A watch should only be polished two to three times in a lifetime. Polishing will take away material from the watch and will alter the shape of the case. The watch will not only look different, but it will decrease in value a lot.
Most watch manufacturers say a watch should have service every 4-5 years (except for Omega, the new co-axial calibre has a service interval of 9-10 years!). Personally, I would only take a watch to service if it is running slow or over 10 seconds to fast every 24 hours. It is more likely the watch will not perform as good as before if you take it in when nothing was wrong.
Also, it is better to frequently wear your watches or at least have them running. Mechanical watches that don't run often are more likely to gain or lose time.
+ 1
Was told exactly the same thing by SA & a friend who collects watches. Only bring in for servicing when the watch is not running well. That's why I don't mind the high servicing cost cos it rarely happens!