Both keeping and violating alliances are legitimate strategies. I don't consider anything said in 1901 to mean anything, because you're almost always going to hear the same things then. From 1902 onward, it's usually best to cozy up with someone who either:

1) is going to have a much harder time than you getting SC's so that you're able to defend when they go for the stab

or

2) Has a slightly better position than you so that you won't be far behind when you go for the stab.

I think he means as a reputation across games. Which is technically metagaming, but you being reliable doesn't make you the metagamer, only those who recognize you are reliable. And, even that isn't considered unacceptable. Knowing your enemy and using their tendencies against them is completely legit. The only thing not legit with regards to reputation is if someone carries a grudge across games. But that is hard to prove.

How do you not "carry a grudge"? Am I so defective as a human being that I can't play this game? I can't imagine opening my belly to someone that has stabbed me in the past.

That's *not* carrying a grudge. Carrying a grudge is intentionally targeting them because they stabbed you in a previous game. Basically, letting anger at the player rule your decisions is holding a grudge. Learning from your mistake and not turning your back on them is just smart playing.

@SuperSteve,
I'll give him a personal example if Draugnar doesn't mind.
In a game that was finished recently between D and me, he had arranged a three-on-one attack on me in 1901 (after I thought we had agreed to an alliance or at least a NAP). Well, a few years down the line, we came to an agreement and actually worked together to create a stalemate. Now I could hold a grudge against him and try to sabotage him in the next game, but that does me no good. He did what he thought was best for him and that didn't work out as well as he thought, and later we both realized that we had more to gain by working together than to recall past offenses.

To me, one of the thrills of the game is taking a calculated risk and opening yourself up to someone you know is untrustworthy. If worse comes to worst, will I be able to get Trieste back? Surely Italy will understand that stopping the Juggernaut is more important than taking one Supply Center from me. Even though I know he's a bit of a greedy bastard.