Preventive maintenance or wait till it breaks?

The main problem is following: you replace 20 years old capacitors that can work 20 years more to new ones that can work just for 3 years. Sometimes old parts had much better quality than same new parts.

I have my tape deck serviced every three years or so. Capacitors were replaced once, ten years ago, transport overhauled, other things done. I send it to Willy Hermann.
Aside from the deck I don't do anything, but I don't have particularly valuable equipment. I once talked to Redgum people, I have their 20 years old integrated, and they said leave it alone for at least next ten years. Fine, sounds good to me, it works like it should.

Keeping heat sinks and ventilation openings clean and clear will do more for your amplifier’s longevity than replacing capacitors.