I believe it has more to do of either the station or (at smaller stations) how well they know you.

YYZ almost always upgraded me.

YOW/YHZ/YYT would not.

After a year of few and far between upgrades I just gave up with hertz and have gone to national. Only once was I not upgraded; and was offered an additional 15% off for the inconveniance.

Most times it is a multi car upgrade (Pay for ICAR and get a Mustang GT 5.0, Edge Sport, Outback SE, FLEX etc.)

Another big difference between hertz and national is nationals cars are always very low mileage; hertz are usualy mid to high mileage.

And since National status matches; vote with your wallet!