There really are no rules so just play around with both and use what sound good to you.

As the prvious post states you can move things around and get a more spacious sound.

Mono more is direct sounding.

Best thing to do is listen to some CD or records. If you have a Mono switch on your stereo compare the diffrence. You can also use the panners on your mixer if you have one.
A great example of Stereo and Mono is the Beach Boys Petsounds release with both the orig Mono mixes and the newer Stereo mixes on the same cd. You can compare the two pretty easy that way. Any old records or CD's will do. It is better to check out some true mono recording.