Steve, I think the complication with the walk-in exit permits is they're available for a given exit date whenever you receive your entry permit. So if you have a permit for, the Onion Valley trailhead for a 5-day trip, the Trail Crest exit permits for your exit date could have been gobbled up by groups on 6-day or 7-day trips. But you're right that they are flexible and they (at least used to) have some discretion with moving dates around a little bit.
Here's a related question: I was under the impression that the Trail Crest exit permits are only required for trips originating from eastside trailheads. Is that true? If so, an exit permit would not be required for a trip originating from Roads End, Zumwalt Meadow, around 54 trail miles to WP including the Whitney summit.
