On big-l, small-l. If you usually vote for Libertarian candidates, when given the choice, I'll probably call you a Libertarian with a big L, even if you don't agree with all of their planks. In the same way, I wouldn't mind being called a Republican, as I usually vote for Republicans.
I sometimes call myself a small-l libertarian, even though I am closer to what we currently term a conservative. But this is because libertarian principles are a significant portion of the current "conservative" ideology.