I personally would always sent to the authorized service centre. I would rather have that peace of mind the parts being replaced are original. Sure other independant service centre may be cheap at least 50%, but I will always have this nagging feeling of "ori or not"?
As for the frequency of servicing, my Rolex 16233 was previously serviced in Singapore way back in 2000 after that it was serviced again at Wisma Dion sometime Sept 2013. Cost me 1950 to overhauled it plus replacement of a new crown (as my old crown has a dent on it) + the usual polishing. Hey!! not bad after 13 years of usage. Can surely last for another 10 years ++. My Rolex 116613LB would probably run for the next 5 years before its due for servicing. Now it all depends if the owner can last that long...

Oh, when they checked my watch, they verbally informed me that my last service was done in Singapore in 2000. Impressive.
Peace...