outside corners should be cut on a 45, inside corners coped. run one side tight to wall cut the next piece at a 45 degree with the face on the short side and the back on the long side. then follow the out line with your coping saw. 45 degree inside corners are box store approach. all inside corners should be done with a coping saw, this way your miters will not open and will stay tight. BOB.
No offense if I'm over-simplifying, but a miter saw is used to make these cuts. Bob's definately right about coping inside corners. It is a must for a professional-looking job. Inside corners of base shoe should be coped as well.