It seems that tactical voting isn't quite the phenomenon internationally as it is in the US. I could be wrong, but I'm not sure if any other countries have two parties that dominate the vote the way the Dems and GOP do in the states.
I have voted tactically, but only because my second-choice candidate had a better chance at beating the incumbent than my first-choice did. I don't know if I would ever have to vote for my third-choice, but desperate times call for desperate measures.
Ha! I had coerced you to vote NDP
Personally, I would never vote tactically. I have always voted for the candidate of my preference. Although in municipal elections, I might ignore some of the fringe candidates who I might prefer more in favour of a more well known candidate. But that didn't stop me from voting for the crazy lady in the last mayoral election.