Why you ask?
Mainly since the oven heats the whole engine, including the cyl bore. I really don't want much distortion going on there other than what is induced while running. Using the torch keeps the heat very localized to where the bearings are and not moving the rest of the engine.

It may or may not make a difference but in my work I use heat for straightening shafts often and have a good idea of what heat can do and it just makes sense in my small brain.