Only, only when a horse is on a lunge line is it acceptable to use side reins, whether there is a ride or board or not. It is dangerous to use side reins when the only one attached to the horse is the rider as the rider cannot adjust them/release them from their position in the saddle.
Draw reins, de gouges, etc.. are designed for use in the saddle.
I like to lunge in side reins, properly adjusted, as they help the horse to learn about contact. When riding it is important for the rider to learn to use her body and "rest" her hands on the withers to learn about contact herself, or to teach the horse. Going "round" does not come from the hands - it comes from the hind legs of the horse, and holding the horse's head down with over tightened side reins is poor horsemanship, whether on the lunge, or especially when riding.
They say money doesn't buy happiness -- well happiness doesn't buy horses!