[Mike I]

You'll notice reduced performance long before you start seeing St Elmo's fire...either way, wires are likely shot at that point and won't meet the resistance spec, so...

Another thing, a good quality set of wires won't arbitrarily go bad simply by transmitting electrons they're designed to transmit. Something has to intervene to cause resistance to increase, such as excessively high heat, abrasion, pulling on the wire instead of the boot to remove from plugs, etc. If none of those conditions are met and the resistance is in range, what's to be gained by replacing them?

Me personally, I don't replace them unless I suspect they've gotten damaged and are no longer performing as they should, however, nothing wrong with replacing them as a time compliance item as Bob recommends.