Another thing that is perhaps a good practice (but I haven't heard it suggested) is to proactively replace heaters every few years.
I roadraced motorcycles for 7 years, and whether the helmet touched the ground or not, I got a new one every other year. It just makes sense... Parts fail with age. Beat the MTBF.
Now the question is, how often is often enough?!? Every 2 years? 3 years?? I have one heater that's a year old and another that is 2 yrs old. I do have the fall-back protection of the controller managing them, but they could still fail OFF, or leak voltage at some point...