Hi, Pam. I'm not trying to take Wick's place--just be the second shift.
Most corners in 15 year old houses don't meet at a perfect 90 degree angle and the same is true of brand new houses. That's why for crown molding and chair rail you want to use coped cuts for inside corners. They are a lot more forgiving than miters and the instructions are on the Delta site that Wick gave you. To do outside corners right, put a bevel gauge on the corner and check the angle with a protractor. The protractor you have left over from high school geometry class is OK if you measure very carefully. You will probably find that the corner is a couple of degrees off 90. You can compensate for this with a good power saw. And you want to cut the molding just a little long and push the bow into the wall or sneak up on the exact length with the saw.
If you are painting you can use a paintable caulk as a filler. If you are going to stain and varnish, you really don't want to have to use a filler.