I tried entering "Toronto to Saguenay" using google, and it offered a google map with 2 routes, one high, one low. The lower route uses the highways you mentioned, mostly TC east around Montreal, and crosses back north at/near Quebec City towards Saguenay. I'd probably use 132 east towards Quebec City from Montreal, but that's just because we've driven it so many times on our way out to Gaspe. It's a slower drive, but more scenic being beside the St. Lawrence River more often.
If you use a GPS, what routing does it choose?