It depends on what you mean when you say "ID". Yes, you must show your license at the gate, but it's extremely rare for gate agents in the lot to ask for company ID for eligibility. On the other hand, I've heard of a number of instances where the person was preferred but they were asked to go to the counter first and then asked for ID to prove eligibility there.

Bottom line, if you've made it to the parking gate, you're probably home free, but don't assume that you will never be challenged for ID just because you are Preferred.