I would say hardly ever, but I would be willing to listen to stories about specific extreme cases that might convince me yes, sometimes. But one thing I can say definitely is that it is not nor should it be a "personal decision," as I have heard even some otherwise thoughtful Christians say lately. The greater society must somehow have some say in whether a person's life is taken (or, if I have to say it, "whether a person is allowed to die") otherwise there will be all sorts of injustice perpretrated. (Think "life insurance" and "inheritance.") At the very least, we have to have some set of criteria set up such that we can say to the family, "OK, everything seems to be on the up-and-up and you guys seem to be reasonable, good-hearted people, so we'll let you make the decision," or, "Nope, sorry. We don't trust you. You can't be the ones to decide to pull the plug." (And, as you know or can guess, I don't think the Schiavo case has come anywhere near an acceptable level of certainty.)