Now I'm not an engineer...
.. but I'd say it's got to be better to strip one gearbox done, go through it properly and then re-build.
That way in the long run, you know everything is O.K. because you've done it properly.
Does that make sense? I'd not have the confidence to open up a L/R gearbox, but I think most 2ndhand boxes will have a life-time's wear in them now.