For me it sorta' depends on the peolple I'd have to be involved with. Some are completly incapable of having a civil discussion no matter what the topic. Of course, as far as vaccinations go, I find that (in general) the people who would argue against me don't have enough critical thinking skills to engage in intelligent discourse. At least, I haven't met one yet. I suppose it's possible.

One issue I tend to steer clear of is abortion. I have a very confused set of ethical issues in regards to that entire debate, and I find plenty to dislike from people on both sides of the argument.

There are times (and this is on other furums, not here) where the people I would be arguing against are so ill-informed, so incapable of understanding logic, and in general so friggin' stoopid that I just cant be bothered to respond, even if it's something I feel strongly about, because it would be a total waste of time.