My reputation never goes below Spotless either. Why would it need to? I still find I can choose whether to keep the peace (for a spaceship game), or taunt the others to declare war (for a world domination game).
Making peace and giving all the other civs all your techs is useful when you're trying to go for a really early spaceship launch, as you have seven tribes researching for you rather than just one! However, I find peace is only really important when you want the extra points for your score in the endgame.
But the underdog - that is, the least developed civ scientifically - seems to be there for just one reason: to take over all except one city and keep as a pet for the endgame!
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"Cattle die, kinsmen die, one day you die yourself;
I know one thing that never dies -- the dead man's reputation."
- Hávamál (the words of Oðin, the High One)