I think commercially, they were often painted on the inside. Probably to help prevent moisture damage to the cab, or simply if there were gaps, it would not be noticeable with the whole inside was painted black. I'm just guessing at that, though.
Do you plan on showing off "under the hood"? If you just have all your stuff crammed into the cab, you might not want be showing off the insides, so it wont matter as much. If you are putting good effort on the inside as well, paint it. You will have people who are interested in seeing the heart of the beast.
