Depends on what kind of snow tires we are talking about. I suspect that the Jeep has the old-fashioned kind that are effective in snow simply because of a very aggressive tread pattern - these work great in dirt-mud too so would be appropriate for a 4wd Jeep. These are pretty imune to tempurature changes. Modern snow tires that get their grip through very advanced tread compound will pretty much turn into bubble gum in warm weather and wear extremely rapidly. The rubber compound is engineered to stay soft at very low temperatures. Bridgstone pioneered this approach with the original Blizzak tire. As a concrete example, my roommate had Dunlop Graspics on her Volvo for three winters, they had plenty of tread life left - maybe 60-70%. She was forced through job circumstances to continue using them through last Summer, by Fall they were down to the wear bars.
But as to the OP's question - in the cold, I would not think there would be a huge difference in wear between using modern snow tires on a cold dry highway and a snow covered one. Some difference, if for no other reason than the fact that if the highway is dry you are likely driving a whole lot faster! But get them off when the weather warms up.
Kevin Rhodes Westbrook, Maine (lots of snow tires over the years)
