The goal of mixing is to create a final product that sounds good out in the real world. It doesn’t matter how “great” it sounds in your studio if it doesn’t translate well in different environment and speaker situations. One way to ensure you aren’t relying to heavily on panning and stereo for clarity and separation is to reference your mix in mono.
The Quick And Easy Test
As your mix starts to come together you want to simply take a few moments and listen to your mix summed to mono. This will give you a good idea of the strength of your overall volume and EQ balance and might expose some weaknesses in your mix. That’s a good thing because you want to fix it before the track leaves your studio. Mono is your friend people!