I am flying from Madrid to Montreal (or possibly Stockholm to Montreal) in early January and single tickets are much more expensive than returns, so I am thinking of utilizing the throwaway ticket ploy. Is there any website out there that can find the optimal return date (i.e. the return date that gives the lowest total price)? Or does it usually not matter?
I'm not aware of any fare search engine that targets precisely this option, but quite a few let you search date ranges, eg. Adioso and ITA Matrix. Rome2Rio is also handy for getting a feel of possible prices, and it's the only search engine I know of that will actually let you search for "Europe to Canada"! (Best offer today: Stuttgart to Montreal, $490.)
In general, 3-4 weeks is the sweet spot for cheapest ticket length. Less than 14 days, and you'll start hitting some fares' minimum stay requirements; over 30 days, and you'll start hitting some fares' maximum stay requirements. Obviously this depends on airline, destination, time of year, phase of moon etc, but since there's not much going on anywhere in late Jan, this will likely work for you.