My personal criteria = if the bike will not hold a straight line, no-handed, it is not a keeper - regardless of tire width. Conditions = no wind, flat road, slow-ish speeds.
edit: the no-handed test is done on a 'bare' bike - water bottle permitted, no baggage.

for me, on a drop bar bike that regularly descends twisty roads at speed - it's a test that everything is as it should be. Confirmation that frame, headset, fork and wheels are aligned.