I clean everything, especially contacts, at least once a year. I replace tubes when it seems like they need it. Anything more than that, well you know the saying "if it ain't broke..."? There's reasons that advice has such staying power.
Preventive maintenance or wait till it breaks?
This is more focused on older gear- do you run gear until it's showing obvious signs of problems or do you do preventive maintenance before problems appear? I fall into the preventive maintenance side as I replace capacitors that are 20 or more years old.