Thermometers are just eye-candy and really not good for cooking, in my opinion. I like to put mine in so the probe is sitting at rack heights as this makes the temp a little more relevant....tho no more accurate.
Another thing I do on my RF is put one at far left and one at far right. The temp is irrelevant but it will indicate a hotter and colder side so you can tell of your pit is heating up or cooling down. Don't think this will work well on a standard offset as the firebox side is always going to be hotter.