I can say in my experience using hot or cold water hasn't made a single difference to the foam or how it foams up/clings. And i've tried Meg's, Cheaper stuff and Valet Pro PH Neutral.

I wouldn't rinse the car first either, as this will dilute the foam and make it slide off easier and quicker, the idea of foam is it stays on for as long as possible to soak and lift the dirt away.

Unless ofcourse you have half inch of mud up side of your car, probably better to blast the bottom half then...