I have no outstanding permit reservations, so can't answer this. Maybe someone with a reservation can try it, and let us know.
I am not sure what "confirming" a reservation does. You still need to go to the Visitor Center to pick up the actual permit.
In years past, you needed to show up by noon the day before for day hikes, and by 10 AM on the date of entry for overnights, to get your permit. After that, they were given away as walk-in permits.
If confirming does anything, it might delay the pickup deadline. But I doubt that is the case. I do know that if you will be late, you can call the Inyo N.F. Wilderness Permit number and let them know, and your permits can be held for later pickup.
