How Often to Run Old Systems to Protect Power Supplies?

How often do you recommend running old computer systems & peripherals (and other old electronics too) to protect the power supplies and other components from the "escape of magic smoke" when next powered up? For instance, I've got computers, peripherals, oscilloscopes, and "home electronics" from the early to mid '70s on up to 5-10 years ago. I've only let the magic smoke escape from one systems power supply so far, but how often should they be powered up for "preventive maintenance"? I'm guessing that the older systems with linear power supplies will need an attempt at reforming the capacitors with a Variac (or equal) before starting any "preventive maintenance" power-ups. After, at least, a visual inspection of the capacitors for damage is performed. What about the systems with switch-mode power supplies? Is there an easier way than isolating or removing the capacitors before "reforming" attempts are made? Any recommendations? I suspect I'm not the only one here with old systems that haven't been powered in years. Thanks, Bob