I haven't stopped being friends with someone who holds an opinion opposite to mine, but that's not to say I never would. There are a couple of people on the fringes of my social circle who I've consciously not made friendly overtures towards because they're forever publishing rants on their LJ about things that I feel strongly about and that I have views diametrically opposed to theirs on, and even though they seem very nice otherwise I don't feel it's worth the hassle.