I definitely dont show up for the rental without insurance already lined up.... whether thats confirming with my credit card company that I'm covered, or buying through MPI.
The thing with Air Miles is there's always a fee on top of everything that you pay... it isn't just allocate miles to rental, drive away "free." They tell you those fees when you search on their website.
Whether Air Miles or not, I use Expedia as at least a starting point to get an idea what an average rate is across the different companies. If from there its obvious that the net dollars I pay to Air Miles is a good deal I'll book there. If not I'll target one or two of the companies that look to have better rates and check online for if they have any promo codes out... then go direct to their company website to see if there's a better rate with those promo codes. Also sometimes also you'll find that when booking direct, if you're willing to pay in advance up front they'll offer a better rate. Sometimes you might also find out that the rate on Expedia turns out to be the best.
Good luck!