some really good comments here, but i would like to add something. i have had enough of my guns blown up while trying to push them into position, so my advice is this:
do not set up guns where the enemy can see. they will make that a priority target. you should have some idea where u want the gun, before you click go and start playing. look at the map before u begin, and try and determine where the best fields of fire will be, and where your gun can best cover the approaching, (or static) enemy. look for places that the enemy wont be able to see at first too. set up an ambush, if you can rush in some gun to cover a road you know he will come down...thats perfect. if they are waiting for you and see it coming, its already too late.
i find (obviously), that its best to find places for your guns before you start, and to not move them.
i played a battle some time ago where i had 4 towed light AT guns, and 4 towed medium AT guns. i moved them up like an infantry advance...first one group, then the other, covering each other. in the end i think i only lost one of the guns. they didnt get alot of kills in the fight, but they did prove very usefull for covering fire...they kept some of the german armor pinned down till i was ready to deal with it. very useful indeed.