I own 4 vintage U87s from the 70s among other vintage Neumann, AKG and Telefunkens. But those are the main ones I use for voice-over and they get a lot of mileage. I'm thinking of sending them in somewhere to get checked up on. They are doing fine but I don't know if a routine check-up is necessary. I know of some recordists who have never sent their mics in to be looked at.
How often do you send your microphones to be checked up on?
Do you keep a reference file to refer back to for it's frequency response?
How do you take care of your mics and what maintenance do you do to them?