The single speaker is much better than switching your monitors into mono for sure. Something like the Avantone mentioned here is really great. The small speaker down at low volume is very revealing. Once you get your mix sounding nice on that it will sound great up loud in stereo on your main speakers.
I prefer to set the panning and get the stereo image setup
before going into mono not the other way around. You will find that panned tracks are still audible in mono but you may just tweak them a little so they are still clear. Then they satisfy the mono speaker
and up in stereo on your mains you will find they have not changed much.
You still need your main speakers up loud for checking bass end and reverb levels though. The small speaker does not give you that information so well.
I spend most of my time on the small mono speaker. Not the other way around.
There is usually a stereo headphone output somewhere that will be carrying your main mix in stereo not being used and it will be great for this job. Sum the outputs properly though through two resistors is better than shorting L and R together. (or some cheap passive mixer) That is not so smart because you are loading down one side with the low output impedance from the other side. You might get distortion doing it that way.