KatanaPilot,
If you are planning to do the Dempster, the Steese or one of the other long stretches of gravel road, extra spare tires are a good call. The Alaska and Canadian portions of the highway is now all paved, as is the Cassiar and other major Alaska highways. The days of busted windshields and flats being a sure thing are now (unfortunately) over so I would not be too worried about duplicate spares. There are a lot of other potential uses for that rear trailer rack (like a couple of jerry cans) and I would be interested in what you are coming up with. The last five times I made trip (four from Western WA and once through Edmonton), I did not experience a single flat despite quite a bit of off-pavement travel.
If you have not done it yet, it is a wonderful trip!