I think a single servicing is adequate and should protect the manufacturer's regular service warranty. I do mine in the late summer/early fall. Heat pump and 90% gas furnace. You could also do it in the spring/early summer.

I personally think twice a year is overkill and probably a sales job for an additional service call. Unless the manufacturer's warranty specifically requires it, I would do annual service only.

When my customers ask me how much a tune up service call cost, i tell them it is free. If you don't have your equipment checked and tuned to get your eff. back where it should be then the extra fuel it will use this winter will be more than what the service would have been, so the service pays for itself. paulbm