I've had the final drive fluid changed twice in 12K miles BUT

The splines/gears [ not sure of the correct terminology ] at the rear of the drive have not been checked to see if the boot has let water in. I don't ride through streams, it saw several days of heavy rain last year heading to Ak. but that's it. 23K on the odo

Should I have my mechanic break it open and check the gears for rust issues. It's been bugging me since I saw one whose gears were completed rusted but had not caused problems for the owner [ yet ].

Is this something I should have checked while it's in for oil change/plugs/valve adjustments? Or is it, as my mechanic states, something rarely checked on a scheduled basis?