Hi,
Our interior is varnished and I do a light sand and recoat about every 5 years. If the teak is oiled I would likely add a coat of teak oil annually, but not too thick.
I think the main thing for the interior is to clean thoroughly to remove grease in the galley, crumbs on the salon floor, hairs under cushions, etc, then if not using the boat during winter, remove cushions, books, food, blankets, etc. and store them in a dry, warm location. Winterize the engine, plumbing, etc and then ensure good ventilation throughout winter. I use the Stor Dry flat silver dehumidifier which apparently circulates 800 cubic feet of air per hour, which I think reduces the likelihood of mould and stale smells in the boat. It's important to check the boat regularly for water leaks and any other developing problems to avoid nasty surprises in the Spring.
Frank