Personally, I only use heat on difficult to sand trim. Scraping and sanding is a better method to remove loose paint and produce a nice foundation for the primer and paint. Using heat can and will remove multiple layers of paint and is a good alternative for hard to sand trim, i.e. – fancy trim on Victorian style homes.
If you have to use heat use a heat gun not a propane torch. A heat gun is a very powerful "hair dryer" that doesn't use an open flame. It is safer than a torch. Using a heat gun can be a bit tricky. It has the ability to burn the paint and catch the wood on fire. Plus, using a heat gun gives off fumes.