Are glow plugs to be considered a "regular" replacement like spark plugs on a gasoline engine? IOW replace at regular intervals? Or simply on an as needed basis
Assuming all are the same age, yes. The thought being....the others can't be far behind the first failed unit, they aren't expensive and you are right there doing the work anyway.
I wouldn't do that with 'injectors'.....but glow plugs, yes.