Newbie Question: Polyutherane All Four Sides of Basement Baseboard?

Subject pretty much sez it all. I am putting baseboards into my basement which is at grade is dry (if not very dry). On the second and third stories, I've never seen the humidity above 40%.
So I've gone through and stained three sides of the baseboard assuming the stain is just for cosmetics in this case. Is this a valid assumption?
Minwax's can suggests/recommends that I polyurethane the wood now. Should I polyurethane all four sides or can I leave the backside, i.e., the side up against the wall, completely untreated?
Am going to be putting baseboard into a ventilated bathroom but am planning on polyurethaning the back side of those baseboards.
Comments?
Thanks all.
~Tom
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no need to poly 4 sides of baseboard in a well ventilated bathroom. the basement would be ONE place I'd go the extra mile if you think there is a potential severe moisture problem; otherwise skip the extra steps on that stuff too. YOU aren't building a piano... :)
dave
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