I agree with Byron that the bottom side is not the source of floor rot, though I have painted the bottom of my floor white just because to me it looks nicer. (I know that I'm the only one that sees it.)
I wish the top side had been painted or resin coated. Where ever I can I've used a can of spray paint
the top side, like inside cabinets and part of the floor under the dinette.
We had a pipe connection fail one night and water came shooting out. We cleaned it up quickly but there was a real difference between the painted areas where water just sat on top of the paint
and the unpainted spots where the floor started to absorb the water. We took a hair dryer to the unpainted sections to completely dry it.
When we eventually get new carpeting, it's my plan to paint the whole thing before carpeting.