By the end of DS9, the Klingons definitely came off as volatile, restless enemies who chafed under peacetime and might easily break faith. I didn't get the sense that they were still active conquerors before the Dominion plants stirred things up against the Cardassians...That was a problem introduced by the shows' writers. I mean, TNG: "Heart of Glory" established that the Klingons who still clung to the old warrior ways were considered outcasts and criminals by the Empire, and pretty strongly implied that the Klingons were actually UFP members. But then Ron Moore and other writers came along and played up the whole "samurai Viking" approach to the Klingons and treated them as basically still warriors and conquerors, making it incongruous that the Federation was allied with them.
But in some ways, the idea that the Federation would ally with them in spite of being more of a warrior culture kind of extends The Undiscovered Country's themes, doesn't it? "Do we have to lose our cultural identity in order to have peace with you? Will you only accept those who are just like you? Or is there room for us to exists side-by-side- and even fight back-to-back if threatened by an outside power- even if we remain true to our culture and very different from yours?"
It seems to me that the idea that the alliance is a compromise- prolonged (and perhaps excruciating to them) peacetime on the Klingons' behalf, accepting these unruly allies in spite of their less-egalitarian, more combat-oriented society- that doesn't require conformity but still exists in their ideologically-different diversity, seems like a very Star Trek-y one, to me.
(If they were actively conquering other star systems, that... yeah, that would be different... but when that started happening with the Cardassians, it did lead to hostilities with the Federation, which suggests to me that nothing like that was supposed to be going on beforehand.)