[Jaydee]

If you run your car for 20 minutes or more, release the pressure, you'll find that for the rest of the day no more pressure will build up, unless you have insufficient cooling system, or the ambient temp is high. I ran my Territory in 43oC the other day, and removed the cap. no pressure. I have a heavy duty radiator.
Another note, if you run no thermostat, the water flow can be to fast for the radiator to actually cool down the coolant. And in winter, the engine will run to cold to be efficient.
JD