Whether the rate is guaranteed, depends on the sort of rate applicable for the CDP! As far as I am aware, there are two general types:

1. CDP's that give a % discount off the standard public rate, such as those associated with airline FF programs, or credit cards. Reservations for these state that rate (i.e. base rate) is guaranteed, but the taxes and charges are not, so the total can change. In my experience, I have always been able to hold them to the total.

2. CDP's that give a negotiated rate that does not change for the period of the contract (usually a year), i.e. a fixed base rate no matter what the public rate is/does. Reservations for these clearly state the rate is not guaranteed, as when the contract is renegotiated, the rate can change. Hertz will not disclose the end date and will let you book the rate past this date, hence the rate not being guaranteed. Avis do it differently - reservations past the end of the contract period default to a high rate.