No. I would not limit the information to just reading the plug and would add engine temp, smoke produced, and performance of the engine into the equation.
Checking the plug is after the fact.
If you run it too hot for too long and then check the plug, the damage could already have been done....
I also wouldn't suggest using the glowplug as your only tuning method. As TGL said, temp, smoke, sound, power output and glow plug are the tools we have for tuning. Using just one of those isn't enough to get it and keep it running right.