I am one that violates the "no hanging on outside walls" rule of thumb... since that was pretty much the only place I had available to hang my Guilds. Here is my setup (my most recent Guild family pic):
I actually have used a thermocouple and several hygrometers to try and measure temp/RH gradients associated with the wall... and have not really seen anything that I would consider an issue (although I do see 3-5% differences in RH around the room, with the driest spot being near the furnace duct). However, I will point out that (1) I live in UT; (2) I use a room humidifier year round, shooting for ~40% RH in the driest spot (and <45% in the most humid), and (3) most of the wall in question is below grade (just guessing, but the headstocks are probably just about grade level). There is also very little heat in the room (due to the tortuous path of the duct work, very little of the forced air actually gets to the room... temp bottoms out at about 60F in winter. I dress warm when I go down to play.). The cheap blankets (<$5 each at the import store) were put there both to negate the temp difference from the wall, and to prevent me from banging guitars against the wall.
I have had guitars hanging on that wall for at least the last 5-6 years, and have not seen any problems... no evidence of condensation or moisture on the guitars, no evidence of any cracking/warping/etc, and tuning is rock stable. The tuning may drift 5-10 cents as the seasons change (temps ~60F in winter, around ~75F in summer), but the guitars stay in tune with themselves.
I use Hercules hangers, which I really like, attached to a backer board. Initially, I had secured the backing board with a dozen toggle bolts into the sheetrock (the studs were just not conveniently located). I think that actually would have been sufficient, but I added half a dozen screws into studs (where I could find them), in response to pleadings from concerned LTGers.