"if I label myself as "conservative", then anyone who doesn't believe exactly as I do on every single issue must ipso facto not be a conservative."
I visualize two amorphous and shifting constellations of beliefs and proposed responses to circumstances. Sometimes the beliefs involved are not economic or political, but moral or philosophical, and most people tend to mix and match.
For instance, a person may accept liberal economic and political notions, but think himself a centrist because he supports the death penalty and thinks the pre-born should only be killed in cases of rape, incest, or danger to the mother.
Another person may be conservative economically and politically but regard himself as a moderate because he supports affirmative action and special privileges for victims of same-sex attraction disorder.
In my view, a label of conservative or liberal becomes useful when a person accepts X number of the points in one constellation of beliefs and rejects the corresponding beliefs of the other. This is not to say, of course, that there is any agreement on what that level is. I'd tend to say that 70% or better is clearly over the line, but others might disagree.
And, of course, the thinking of some people seems to be such a jumble of the two as to make it impossible to put them clearly in one category or the other.
However, a person is unlikely to be accepted by those who fall solidly in one or the other camp unless he accepts their view on the "deal breakers." Liberals are not going to accept you if you hold that abortion is never permissible, or that women should not work unless it's absolutely necessary to sustain life. Conservatives are unlikely to accept you as conservative if you favor eliminating the age of consent, or high taxes and cradle-to-grave welfare.
All in all, I think it most useful to draw the line between those with a utopian vision and those who, as Goldberg says, recognize that everything is a trade-off.