I personally service the car based on km's travelled (or if I have been driving it like a madman
). In the extra couple of months it may take to reach the desired km's for a service, an oil or a component will not suddenly deteriorate causing you any issues.
Out of interest, my parents Merc tells you when it needs to be serviced, no more services by km or time, it just lets you know on the dash when it needs one. So I think km's and driving style play more of an importance on when to get the car serviced.