As a 17 year assoc. at Wal-Mart I have some info on this issue. Many if not all of these purchasing issues change according to issues within a specific store. As stated in previous post theft issues or specific issues in a community (request by customers, city officials, police etc...).
The vast majority of stores I have worked in required purchase in SG of most types of ammo. The exception in most cases would be shotgun target ammo and slugs. That is the standard.
Guns are always walked to the door, except in some cases black powder weapons which in some instances (mostly in the past) where black powder weapons were availble on the counter and not kept in the back(locked up).
When I worked in Virginia we were not allowed to sell a weapon and ammo in the same transation. I do not believe this was a law, but a policy we were asked to follow.
I have never heard of a situation where ammo was required to be walked up front unless associate was not old enough to sell it. You are not allowed to sell something you are not old enough to purchase (course then someone under 18 would normally not ever man the SG register)