Nix on the microwave—ok, got it! 
I’ve rewrapped a handful of cells. I ALWAYS replace the insulator ring, and use a high quality wrap, because that wrap is what insulates the negative electrode (Most 18650’s are case negative, which means that the whole case is “hot”). I would not use a butane torch or lighter either, a hairdryer, or preferably a heatgun will give a much more even shrink. You can get a cheap heat gun for less than $10. The flame based torches can heat too much and weaken the wrap, in my experience.
It’s all just a matter of understanding and using your heat source correctly. Anything that reaches a temp that will start the shrink will work just fine, as long as you do. Not to be snarky, but if you can’t control a torch flame in such a way as to not damage the wrap or even the cell itself, by all means do not use that yourself! 
But it can be done just fine…would never suggest that someone learn and develop “torch skills” on cells/wraps, though…
^^Been practicing on lots of cigars. But still prefer a heat gun.