Beware of taking statements as fact. Below 200Hz should be mono, as in central is frequently NOT how people do it. It's one of those so called facts that kind of get accepted because of the physics. Because of the wavelength, low bass is extremely difficult to localise. But many people can do it. Also, bass is not a sine wave. There are harmonics that CAN be localised, so panning a bass does shift the apparent location of the higher content. Removing bass from one channel, or even just reducing it can also free up space to allow some other low frequency stuff to be in it's place. Helps avoid a low frequency mess sometimes.
You can hear this going wrong with those compact PA's that have only one sub that goes to one side, in many venues because it is in the way in the middle. You get bass, but definition is poorer. Have two sub speakers and play some early 70s synth stuff and you can hear alternating bass synth lines when they go from speaker to speaker. Try it in your studio, one note left, one not right, going from low to high. There is an effect lower than 200Hz. The rule does make sense, but when starting out you MUST try these things to hear them before you decide to follow them or not.
When I have newly qualified students doing PA jobs for me and you give them the mic box, they always put an SM57 on the snare, it's what they've been taught. But I look at the snare and the room, and often pick something else. Might even me a condenser! rules are made to be broken, when you know better. Perhaps they should be guidelines or recommendations for experiment?
