You are correct about the tension and cold weather.  If installing the high tension fence on a hot day, only pull it to about 75% of determined tension.  On a mild day, 60s and 70s, about 85%-90% tension should work.  A zero degree day will put a lot of tension in that wire.  Where it is steel wire, it won't shrink and elongate like aluminum or copper, but it will still have some movement.