Preston,
Most old growth pine is loaded with resin, and the resin odor is quite powerful. I would be concerned that if the pine were unfinished on the inside this odor would penetrate the rather porous Spanish cedar, lending an undesirable aroma and flavor to the cigars. For that reason, I would finish the inside and outside of the box with varnish or poly before inserting the Spanish cedar. I have only built one humidor, so I am not an expert, and others may offer a more experienced opinion. Also, I am not a cigar smoker, and I am just assuming that a pine resin flavor in a cigar would not be welcome. Of course, anyone who is puffing on burning leaves may find that the turpentine bouquet adds an interesting depth and complexity. :wwink::gar-La;
Hope this helps.
Matt