Usually that's good advice. But not always. If it's still working, you can take preventive steps before an easy-to-fix issue causes impossible-to-fix damage.

Silicon does degrade (and fails). But in my experience, most issues fall in 2 categories:

1) "Mechanical" problems. Connector problems, loose solder joints, corrosion, cracks in pcb traces, etc.

2) Power supply issues. Electrolytic capacitors are suspect #1 there (and they may not respond well to being powered after a looong time in storage).

Rules I apply:

a) If unsure how a machine looks inside (condition, mods etc): inspect internals before powering up.

b) Check that a machine is working before modifying it in any way. If not working: minimize steps to get to a working state.

c) Do mods in small incremental steps.

That way you can always backtrack to last working state.