How to evaluate a functioning rear axle assembly

[gilitos]

I have never had my rear axle apart; I keep oil in it and it's been working fine for many decades. I'm about to start some major work which will include removing the axle from the car: I'm replacing the shackles among other things. How can one decide if the axle should be disassembled and rebuilt? If there are no symptoms, is it worth diving into it?