I'd say having a friendly civ nearby would get in the way of your domination/conquest victory. In my recent game I had a friendly peter nearby (even after he went into free religion) and I completely sucked up to him with defensive pact and giving in to requests while I teched quickly to space. He vassalized either voluntarily or by war the rest of the freakin' world. (fortunately I had a lot of land from early expansion/new world colonization so he couldn't get domination) He had a late game war the other superpower and dang it I have never seen so many nukes go off. His power level shot off to the stars. So I just patted his back as he completely destroyed our world as I get the heck off that planet. Yes, best friends forever! But yeah in retrospect it probably would have been better to backstab him earlier, he could have probably won diplomatically if the U.N. was built.