Acceptable. But it's a tactic I would never use. Reason? It DOES take away from your win. If your only chance was to backstabb, it means your oponents were better players: you just suckered them. Building a friendly relationship, then suckering them, is NOT skill: absolutely anyone can, it simply takes a sort of luck to befriend the top alliance, and that the other alliance gives you the break. Key word, being that they GIVE you the chance, you do not win it by yourself. That's why it's dirty: it's not that you outsmart your oponent, you do not outdo them: they trust you, give you a chance to survive, and you take it one step further.
And Allen, NAPs aren't made to be broken! That they can be broken, aye, but they're not there for the sole, nor main, purpose of gaining time to then strike them before it ends, to surprise the enemy: they're made to pospone a battle that would be inconvenient for both parties, or to keep two alliances with close friendship ties from hitting each other, either for some time or for the whole era.
Btw, there's something I always say against that saying; that all is fair in love and war. The truth is, NOTHING is fair in love and war.
IGN: Vault Dweller