Although my shop is heated, the thermostat stays set on 55F year round (hot water heat in the floor). Even when it's that cold I still don't pre-heat Parks 50....because one small drop point hunter will bring the heat up to 70F....even in the 4 gallon quench tank that I use. If it's really cold, then yes, I would bring the Parks to the 70-90 range....but unless we're talking below zero temps, and the Parks thickens up, I don't think it will make a lot of difference.
Since my shop is heated, I can't say that from experience, since my Parks has never seen below 55F....... just making an educated guess based on my experiences with various quenchants.