Sorry Mr Ghetto, but no professional studio mixing engineer has ever advised me to mix solely in mono.
They have told me to occaisionally switch to mono to check for any phasing issues, as a lot of cheaper radio playback ends up in mono. Likewise, all the movie mixing engineers I speak to constantly switch between 5.1, stereo foldowns and mono for the aforementioned reasons when the movie in screened instereo theaters or at home/on tv.
However, saying it is industry standard to mix only in mono is just plain wrong. It just isn't. I'm sure theres there odd eccentric engineer who has been doing this his whole life and refuses to do it any other way, but it mixing the whole time in mono is dumb.
You mix in your intended release format, and as a secondary goal you make sure it works in mono (or stereo if you're mixing in 5.1).