I use AutoSlash to keep track of rates. I find that if I reserve a Hertz, because I'm President's Club I can choose whatever car is in the President's section, so I just book the least expensive Hertz I can find.
Good point. My employer allows me to use our negotiated rate for personal rentals with National/Alamo, and one of my credit cards includes Executive Aisle membership with the former. It's not always the rock-bottom cheapest price, but it is usually within spitting distance, and the ability to choose a car from the ones available is nice.
For example, the last time we were at Palm Beach for a winter beach week we scored a Mustang convertible for the price of a mid-size.