You mean 4-5pm Saturday, right? There's no autograph session on Friday that I know about.
Not all the guests will be at that autograph session, but all the voice actors should be. If you want an autograph from any of the artist guests, your best bet is to visit them in Artists' Alley. Also, Brian Brushwood will be busy setting up his show, but should be available briefly after his show to say hello, sign stuff, and sell some DVDs and T-shirts.
Guests I am currently expecting to be at the 4-5pm Saturday autograph session are:
* Christopher Ayres
* J. Michael Tatum
* Daniel Kevin Harrison
* Sharon Lee
* Steve Miller
This list is subject to change and guests may be added or removed as their schedule allows.
I don't know who will be at the autograph session that's listed for the tent at 5pm Sunday, but the voice actors will not be. (They'll either already be in the air or on their way to the airport.)
We would prefer if you did not put the guests on the spot and ask for autographs after their panels. There are other panels scheduled immediately after their panels which need to get in and start on time. The voice actors also sometimes have other things scheduled which don't appear on you schedule (like eating or scheduled press interviews.) However, if you see them walking around the convention, feel free to approach them as long as it doesn't look like they're in a hurry to get somewhere.