I've noticed that on this board that those who say they are Libertarian don't seem to support the Libertarian candidate, Gary Johnson. Watching him on Jon Stewart reminded me to ask why. It also reminded me that would make a much better candidate than the Crazy Uncle of the Republican Party.
It should be obvious but if you don't support him, there's not much chance he'll get anywhere close to winning. Of course, there's no chance he'll win if you do support him but why whine that you want a third party and then not actually support the candidate?
As for me, I have voted for the Libertarian candidate on and off. It depends on the candidate and other issues. I last voted Republican in 1988 for HW. It was HW that was the straw that made me leave the party. I voted for Perot in 1992 because I wanted a good third party splash and got one. I liked Harry Browne and voted for him a couple of times. I like Barr personally for holding Clinton accountable for the crimes he committed, but I didn't think he was really a libertarian so I didn't vote for him.
So there isn't a single answer to your question. As for this election. I think Romney sucks but I won't blink when I vote for him because Obama is a Marxist. I think any libertarian who would leave a Marxist in the White House and not for for Romney to remove him has issues.
I've gotten to be more pragmatic as I've grown older...
in that sense, I would encourage persons who do not live in a battleground state to vote for the candidate who most closely represents their point of view, regardless of whether that candidate has a chance of winning the election...
and, for those folks who do live in a battleground state, I would encourage you to hold your nose and vote for Romney...