If the vehicle is still under warranty, you would want to follow whatever established guidelines to be entitled to warranty claims(when necessary).

If the vehicle is not entitled to any warranty, usually cars above 5 years old, then you can do whatever you like. For my vehicle which is already 8 years old, I follow the mileage. If following date, it's usually 6 months interval before you send in the car for service. For my case, the interval for my last service is 2 years 1 month (instead of 6 months) since I follow the mileage which is 10,000 km.