You probably have heard the term "strategic voting" tossed around a lot. Well, the first point I have to make is that the rest of the English-speaking world calls it "tactical voting", and this is a much more appropriate term for it. Tactics are about achieving a short term, clearcut goal (such as defeating the Conservatives); strategy is about achieving a long term, perhaps more nebulous goal (such as creating a just society). Voting for the candidate most likely to defeat the Conservatives is an example of this.
So is tactical voting worthwhile? I'm not sure. Fortunately, for me it doesn't matter, since I live in a pretty safe NDP riding, but what if I lived in a riding where the best chance of defeating the Conservatives was to vote Liberal or Green? I'm inclined to think I'd still vote NDP, because even if the Conservatives are held to a minority, they will likely govern as if they had a majority anyway, thanks to the virtually nonexistent Liberal opposition. After all, they've had a de facto majority for the last two years. And when you vote for the party you most believe in, you're helping to provide them with funds, thanks to the proportional funding rules that federal elections run under.
However, if you do decide to vote tactically, it's important to make sure you do it properly. Don't blindly vote Liberal; check out this website (in fact, it's worth a look even if you've already decided who you're voting for; it's quite an interesting site). You can select your riding (or enter your postal code if you don't know your riding) and it will tell you who to vote for. In non-swing ridings, it will tell you to vote for whoever you like, whereas in ridings where the Tories have a chance of winning it will tell you who has the best chance of stopping them.