Oddly enough, the corners on crown molding are NOT mitred when installed to the highest standards -- they are hand coped instead (or by machine using a really new product..). This takes extra labour, but gives joints that don't open up with seasonal wood movement.

But who uses wood nowadays for painted moldings? If using a synthetic material, or even a wallboard-based product, the joints can be mitred and then filled with wallboard mud compound for seamless joinery.

Cheers