Minor disagreement with tec.
A number of people have observed that when a pitcher is at 100% (rather than, say, 81(100)) on the day before he starts, his fatigue number dips less far after a similar length start; that is, all 100% are not equal with regard to fatigue. Assuming that in-game fatigue works the same way as post-game fatigue, a pitcher who is rested one extra day will perform somewhat better on average, due to less in-game fatigue, than a pitcher who pitches on the first day he achieves 100%.
The reason it's a minor disagreement is that I haven't noticed that a pitcher who has had a long rest fatigues less than a pitcher who has just had a single off-day at 100%. So I don't really think long rest helps. But I feel certain that there are wide ranges of fatigue that all show up as 100%.