Well it is early and I am waiting on the sun to come up so I will chime in.
I think the every year deal is a great big CYA.
How many cargo trailers get serviced like that every year? I have a 16 foot flat bed that has been loaned out for trips like SC, VA, and parts in between. I got all hinky about the wheel bearings after being on this board for some time. So at the 10 plus year mark I bought new seals and serviced the bearings. They all looked good as new when i pulled them. And the brakes were OK also.
Sure once a year will not hurt anything if they get back together right. Is it necessary? Your call.
It is important to do what makes one feel cozy also.