I want to try and make this obvious statement without sounding condescending: There are lots of types of wood and lots of types or products made from wood and depending on what you're using and what you're making, the processes are often going to be different. A guitar top and a tabletop share almost nothing in common other than they can be made of wood - the species, the drying standards, thickness, joining methods, etc....totally different things so you should expect that they are treated differently in the finishing steps, as well. I understand the OP's curiosity and question, so completely reasonable to inquire.
I'll just say that in my opinion, finishing the underside would provide almost no advantages to the instrument and a cost/benefit analysis would 99.9% benefit leaving it raw.
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