Does a 100 year old home need attic ventilation added if no problems have occurred?
This is a subjective question which depends on your prerogative for several things. How long will you stay in the home (or what's your predilection for saving the planet)? Better venting may keep the attic cooler, which may in turn reduce air conditioning costs (financial and ecological). The payoff (in time and pollution savings) isn't clear without a detailed analysis of current conditions and potential improvement.

If you don't have moisture problems now, and you haven't found cooling costs to be unreasonable, I would not make changes at this stage in the home's impressive life. If you feel like the HVAC system struggles in the heat of summer, it may be a wise move. 

FYI, attic vents should remain open year-round. Even in winter there's moisture to be released, especially without a modern sealed building envelope. Much more vapor than you realize may be migrating out of the living space. Also, you want most of the heat to be released anyway to prevent ice damming. You might close them somewhat to retain a bit of heat, but only if it's convenient. If it means pushing into an attic and getting insulation all over, don't bother. Leave them open.