One thing to keep in mind when positioning chair rails is the proportions of the room. If your chair rails will be purely decorative, then you don't need to install them at a particular height. In a small room, a chair rail that is too high will make the room seem small and overpowering. If you lower the chair rail, the room appears more open.
I did what Henry in MI suggested; I watched out for the window sill height. It was around 28 inches, so I removed the window skirt and made the chair rail that exact height so the chair rail becomes the bottom window trim as well. One continuous line around the room, and it's much lower than the standard 36 inches, but the room looks bigger and much more elegant.