Firstly, plug chops are generally done at wide-open throttle.. at top speed. The main jet has no influence on idle mix. The carb has a separate idle system.
That being said, factory bikes are carefully set up to run very well.. Components are chosen with a balance of reasonable power and good reliability, low maintainance and long life in mind.
A plug chop on a stock bike will tell you if something is not right.. Perhaps you're using too much oil in the mix.. or your air filter is dirty .. or the points have worn and ignition timing is off.. Or the weather got hot and a cooler heat range plug should be used, etc, etc.
There's no reason to change main jets to correct some fault with air:fuel mixture on a stock bike. What would be called for is a thorough tuneup that will return the bike to as-new running condition.
Plug chops for high performance on a modified bike are a whole different animal. These chops are aimed at extracting maximum power by leaning the mix to the absolute minimum safe ratio.. or as a way to select or modify different major components.. Squeezing out some extra power reduces reliability and jeopardizes the engine in general.
It's just about pointless to do a plug chop with a used plug.
The idea is to get a single, accurate snapshot photo of exactly what is happening at this very moment under these immediate conditions. Reading an old plug is reading obsolete information. Anyway, there's nothing wrong with having 4 or 5 (practically) new spark plugs laying around..
You have the correct idle air:fuel ratio when the bike idles well.. it idles at a low rpm whether the engine is cold or hot, any time 24/7/365 ..
Again, all this is set up back at the factory. Idle jets are not adjustable or removable on the simpler carbs.
The few (if any) available idle adjustments are all you need IF the fuel system is clean and the bike is in perfect factory tune..