Since I brew in a barn in Ohio, I don't find it necessary to preheat the tun in the 90-100 degree summer heat, but when it's 10 degrees in the winter, I find it necessary to preheat it. But by preheating, I just leave inside next to the wood burner until I need it.
However, I am confused by this paradox since I usually keep my ratio around 1.75 quarts per pound, but I need my strike temp about 18 degrees over the target mash temp with bigger beers that use lets say 15 lb of grain. I run my strike temp 13 or 14 over for smaller beers that require 10ish lbs.