Enterprise/Alamo/National, Dollar, and Hertz will always waive the underage fee if you have a corporate or association contract that stipulates it. Avis/Budget seems to have fewer such underage fee waived contracts. Before Hertz put up the underage coupon, many Avis locations refused the retail underage business.
If you're a retail renter, Hertz is the only one that offers the fee waived everywhere. Certain Enterprise locations do not list an underage fee when you select ages 21-24, but those are very rare and possibly errors.