I've been riding on the street for 30-some years now and was originally taught to ride in the left tire lane, although i dont ever recall it being referred to as a blocking lane. I read this thread a few days ago and it gave me things to think about. Then I went for a nice long ride the other day and paid special attention to the pros and cons of riding in the "blocking" position. For my two cents, the blocking position offers far more pros than cons. Here's why: it was early morning and even though the road near my house I had always considered the same for the entire width, because of the angle of the sun I could see a definite shine in the middle of the lane compared to what was in the tire tracks. That there would tell me there IS more oil or other slippery stuff, and we cant always see it! I came up on a line of vehicles travelling along a two lane highway, that one in front of me was an SUV. If I moved to the centre of my lane, I could not see what was going on in front of the SUV, but when I stayed in the "blocking" position, I could see several vehicles. Since one of the keys to staying upright is being aware of whats going on around you all the time, it makes sense to be able to see the vehicles ahead and not just the one directly in front. As others have mentioned, certainly conditions etc will dictate what is best and it makes more sense that if I am the only vehicle going in one direction and there is a row of vehicles travelling in the opposite direction then i will drift to the centre or to the right tire track because as sure as a duck on a junebug there will be some impatient cager peaking out over the centre line looking to pass. IMO riding in the blocking position is the best choice the majority of the time, but be aware of road conditions and the location of other vehicles, driveways, etc.
BTW, I live east of toronto so most of my riding is fairly rural two lane. All bets are off on the 401 and downtown cuz I've seen some giant craters in the tire lanes! I honestly dont know how you toronto guys ride in that constant traffic without crapping yourself! Every time I take a trip through there I'm left shaking my head at how you can find it enjoyable day in and day out fighting with that traffic mess. stay safe out there.