If you have aluminum heads and aren't having problems, plugs should be changed cold. Once you get stuck, you have to reassess. I have a volvo 4 cyl turbo that had a stuck plug on #3. I stopped messing with it when I realized it was stuck, and started soaking it with kroil, pb blaster, etc every chance I could. I'd run it between soakings to get the lube into the threads. After several weeks like this, I started the cold car, ran it about 4 minutes (feeling the exh manifold to feel when it felt almost hot), then started the two flats out, one flat in, game. I got the plug out without stripping the soft Al head and cleaned up the hole so I didn't need a helicoil.
Sometimes heat can be your friend when you have dissimilar metals, you just need to think about what you're doing.