Professional cyclists will often use an analogy regarding "matchbooks" or matches, when they discuss racing tactics. And this same theory pertains to recreational cyclists as well.
The premise is, that when you participate in a race or an event such as a Century, your body has a fixed amount of energy stores that it can provide. This fixed amount is referred to a the "matchbook" (perhaps 20 matches). Every time you put in a hard effort, you've burned a match. Put in an extremely hard effort, and you've burned 2 or 3 matches.
What professional cyclists try to do is conserve as many matches as they can during a race, and have enough stored energy for that final push to the finish. Cyclists who are considered "climbers", might use up their matches earlier in the race for climbing a mountain, instead of waiting until the end of the race, where they won't bother competing for the sprint finish.
So how does this pertain to a Century rider? You've got your own pack of 20 matches when you start the ride. Each hard effort that you give is going to burn up a match. If you have a particularly hard hill to climb, than there goes a match. You can't avoid the hills, so you'd better keep some extra matches available for other portions of the ride.
There will times in the ride that you burn matches because the terrain is difficult, and there's nothing you can do about it. That's why you've trained for the Century. The more important decision is when to put out a harder effort than necessary; just because you want to keep up with the fast guys, or set a personal best time. This is where you need to be careful - burning matches unnecessarily.
Your best strategy for a 100 mile ride is to consider the fact that your body has a limited amount of energy available, and you need to be careful about how and when you use it. So conserve your matches early on, and use them late in the ride when you know you've got finish line in sight.
The perfect ride is when you end the day with 1 match left in your matchbook. That way you can use it to light your victory cigar!