is there a general temperature I can note?
Not really,depends on the type weather on a even day,wind chil and cloudly days will play a big part.On sunny days I've wash down in the low 20's and also have run into days with temps in the high 30's but freezen b/c of the wind chil.You will know when you cann't wash by the amount of ice on your cap bill
.If night time temps go down into the teens I stop washing the trucks in time for the water to dry around the doors and locks.I've frozen trucks to the ground before where they cann't move,water can get in the brake drums and freeze solid,on the trailers too.Alot of times your water run off will freeze on the ground but not on the trucks so becareful walking on the ice.
Winterize your rig everynight......