It’s a recommended practice to check your mix in Mono. You want to make sure that the sound of your mix doesn’t drastically change when you switch over to Mono. It’s also good to check while you are recording to make sure you’re not ending up with out of phase drum overheads – or some other nasty problem.
You can walk out into the hallway and listen to your mix. This gets you to mono, but it also gets you other reflections you wouldn’t get sitting in front of your speakers. If your audio software or audio interface doesn’t support switching to mono, you can get this free plugin – Monomaker – to check your mix without all the hassle of getting up out of your seat. http://destroyfx.smartelectronix.com/docs/monomaker.html
GarageBand doesn’t do mono output, so Monomaker can be a useful plugin to throw on the master mix while you’re recording and adjusting levels.