I do try and remember to warn clients that the room needs to be toasty and I always ask if there's anything/where I can warm the towels... not because I think warm towels are necessary but because generally in Winter they're always freezing by the time I've hauled them halfway across London to the client's home. If so, arriving a little bit early enables you to warm them a little.
I do, however, put them on the floor of my car, passenger seat side, with the heater full blast with the car heater directed to the feet - just in case.
Hope that makes sense.
x
Henry