Don't ever let yourself think it's safe. That doesn't mean don't do it. Lots of things are dangerous but still worth doing. Just don't get too comfortable and get careless.
A lot of people worry about getting picked up by some fictitious axe-murdered. That does happen. There's a highway in BC where a couple of girls a year have disappeared for most of the last 30 years. For the most part though that's not the biggest concern. The big concern is being picked up by someone who is simply a bad driver, someone who is drunk or high, or someone whose car is not especially safe.
Try to wait at a gas station near the highway and find rides from there. That way you can talk to drivers for a minute before you get into the car. It also makes it easier for the drivers.
Be prepared for some long waits. Northern Ontario is really far too big. Carry a warm sleeping bag and a ground sheet so you can camp out.
I tend to pick up hitchikers fairly often so if you pass through Alberta maybe we'll meet.