Only putting an IEM in one ear is a bad idea. For one thing, the whole point of IEMs is to limit the volume of stage noise hitting your ear drum. If you leave an ear exposed to the room, you aren't sparing your ear anything. Also, I have heard (and I'm not up to speed on the medical basis of this) that your ears can vary their sensitivity to noise and damage (Dr. Google is referencing the stapedius and tensor tympani muscles that dampen reactions), and that if you use an earplug (which an IEM bud basically is) on one ear but not the other, it can screw this up. A sound bud in one ear is for bodyguards and secret security agents so that they can get radio messages in that ear while engaging the room around them, not for something constant like music.
The best solution for over-isolation from room sound when using an IEM is to set up a mic pointed at the room and running it as an "ambient" channel. Then you can blend the room sound into the rest of your mix while still protecting your ears.