What is the general rule for stripping wood trim and wood doors? When is it best to use a heat gun or chemical stripper?
Stripping? Just don’t do it. Seriously, it is a lot of work to strip and refinish wood. I think you need to evaluate the existing wood and doors for their value. Inexpensive pine molding should just be replaced, along with Luan veneer doors. Older, intricate moldings and doors should be saved. Oak and other hardwoods will be much easier to strip and refinish.
Whenever you strip paint from a surface you need to be aware of possible lead content in paint, so don’t strip lead paint without the proper precautions.
The process is messy and tough to do. Heat guns and scrapers work well in some cases, along with chemical strippers. Success depends on the type of paint, number of layers, physical access, shape, and contours. I would try both methods. There are also many types of paint strippers. For a large project, you should test several types and brands. Any doors or parts that can be removed should be stripped outside on saw horses. You need lots of ventilation to play it safe. Good luck!