Indirect fire artillary perhaps, but AT guns and short barrel "Infantry" guns are needed pretty much up with the front lines, otherwise they serve no use. AT guns can't stop tanks from overrunning your infantry if they don't have LoS. And in my experience thus far, if my AT guns have LoS to be remotely useful, then enemy FO's seem to be able to spot them and wipe them out with very short mortar volleys.
In real life, if on the defensive, one would dig their AT guns into gun pits, cover them with camoflage, etc. In the game we have neither of these options. Other than placing the guns in a wooded area, they are completely exposed. So one tactic that you might use to keep them from getting wiped out by mortars before you ever even see an enemy is to have your gun crews take cover in nearby trenches when you see the ranging shots, but since they can't remount their guns afterward, this tactic is useless. Thus, AT gun crews seem to pretty much be slaughtered within moments of enemy contact, unless you position them so far back behind LoS blocking terrain that they are useless to you.
Edit: Every firsthand account I've read of AT gun crews in calibers under 88mm were pretty much right up in the front line trenchworks with the rest of the infantry, usually engaging enemy tanks at very short range. They relied on earthworks and camoflage to keep them concealed and alive until the enemy armor closed in. In offensive engagements they followed a few hundred yards behind the first waves, moving to engage enemy armor as it was encountered by the infantry.