A way to get a good fit when installing decorative molding is to install the first piece with a square end. Then cut the second piece at a 45-degree angle in a miter box or saw to expose the profile. Use a coping saw and follow the edge of the profile, under-cutting it about 3 degrees. This lets you get a good fit, even if the corner is not perfectly square.
Have your building materials supplier check moisture content of flooring, paneling, and trim boards to make sure the material that has been stored correctly.