I used 60/40 in my taurus while going to college in Utica, NY. I was heading home for the weekend after a major snowfall the previous night. With wind chill, it was -50*f. Wind was 30 mph.. I hopped in the car and let it warm up for 10 mins.. Waited for warmth from heater core, no warmth. Started out on the highway to head home, and no heat after 10 miles. No heat at all for an hour, my radiator was frozen solid. There were no ill effects from the freeze, I thought for sure there would have been a crack in the rad, but nothing.
Moral of the story: If it gets below zero in your area, even just once a year, leave room for wind chill temps. I use 70/30 now, but there is a point in concentration, no ill effects in the summer that I've noticed from the 70/30.