Our town just overhauled its guidelines for septic systems in an attempt to avoid having to install a town-wide sewer system and take part in a regional treatment facility.
Current (new) guidelines:
1) If no disposal, pump the tank every 5 years at a minimum.
2) If you have a disposal, they recommend that you pump every three years, and for new construction they require that the septic tank be oversized by one additional bedroom size.
There is no delineation between what kind of disposal you can have, and recent studies have shown that those disposals with injector bio-cartridges or a "super-grinding action" are no easier on your septic than those without.
Pumping your tank every three years is your best insurance to a long-lived septic field. At $150 per pumping (cost in my locale for a 1500 gal tank), it's fairly cheap insurance.