IC, I did a pillar and partial bedding. From my reading there are several types, from full length, full action, pillar only and partial. Since the ATI is mostly holow I went with partial.
I made my pillars by measuring my barreled action metal to metal and adding .050 to the length of the pillar . Also make sure to score the outside of the pillars to allow the epoxy to bind. I bedded mine similar to the pics in this link http://forums.gunbroker.com/topic.asp?TOPIC_ID=288948
To answer your question, I did both the front and rear screws the tang and around the chamber.
If you are looking at the rear guard race of your ATI stock you will see it is kind of designed with play to accomodate minor machine differences. Bedding the back will take any forward to rear play and secure the action to the stock. I also was able to fill in some of the gaps along the action/ stock line. I taped up the mag and trigger holes to prevent epoxy from getting in.
There are some good vids and articles online that explain the process, just make sure to research and follow all of the instructions
I will post pics after I clean up and color the epoxy, and a range report to follow.