I ride/race many centuries (and longer). I typically taper the intensity of the rides, but not the time on the bike. I like to ease up 3-4 days prior to the century - I keep my efforts 'social', while riding the same duration, but shorter distance. I'll usually have a rest day the day prior to the century - that's also the day I give the bike a final clean/tune-up and layout the gear/food I want with me on the ride.

I find that the challenge of century (and longer) rides is the time in the saddle - my legs & heart/lungs don't suffer as much as general fatigue from staying on the bike for many hours.