For the 40k:1 on/off CR, pretty much any room where other lighting can be extinguished to the point that if the projector is off the screen is much dimmer than with the projector on and showing a complete blackout screen. So, even a white room can support this. I mentioned this in my article about CR and a room that was largely off white, but where a CRT with about 700k:1 on/off CR could still show an advantage from that very high on/off CR.
For 1200:1 ANSI CR, not that many rooms. ANSI CR off the screen is generally very dependent on how dark the walls and other surfaces are. On/off CR is not (unless there is other lighting besides that coming out of the lens and going straight from the projector to the screen).