I "inspect" everytime the case is opened. I use a magnifying glass to look at the oring and mating surfaces for sand, hairs or whatever should not be there. I also never open or close the housing on a boat or beach unless I absolutely must. I open, inspect and close the case in a reasonably clean place...usually my room before leaving. I take the oring out for inspection once or twice a year for inspection unless I find trash in the mating surfaces, then it comes out for a complete cleaning and inspection.
Greasing the oring depends several factors. First off, should you even grease the Oring. Some housing, Ikelites for example, do not need to be greased, others which do require grease only need it occasionally, once a dive trip is plenty IMO. Remember, the grease it to lubricate the mating surfaces and the oring as they slide past each other and plays no role in sealing of the housing.