I know people don't use travel agencies anymore as there are so many do it yourself websites available, but with everyone touting themselves as "discount", how can you be sure you're getting the best price?
Unfortunately; the answer is, there is no answer. Just like anything else you need to shop around.
Between the high-profile travel sites, the airline-package specific sites, the discount or last-minute sites, some specialize in one thing or another, but do seem to vary as to their actual lowest price at any one time.
My recommendation would be (which is not the same as an answer) to plug the same travel details into some small subset of sites. Take the one that you like best, and then forget about it.
Its very time consuming trying to find the best deal, and unfortunately there doesn't seem to be a single place where you can find the ultimate best price
Not sure about Canada, but in the UK we have TravelSupermarket.com, Travelocity.co.uk and Expedia.co.uk, the first one being the better. They tend to be better than going to the airline/travel company's website, but then you will miss out on airline exclusives
Another thing I have heard is the timing of when you buy your tickets makes a big difference. The closer you get to your departure date the cheaper the fares sometimes, but you run the risk of not getting the seat you need.
There is no ticket pricing site that will help you with timing.
I have used http://www.kayak.com before, and they aggregate the other sites to save a bit of time.