It depends upon the system you use, if you use the old airbrush then they typically use the circular method due to the pen like hand piece. If you use a professional system then as long as you do it evenly it shouldn't matter how you do it.
We were trained to start from the top and spray down. Keep the gun straight, don't tilt and keep the distance of it the same from every part of the body. So if someone has big boobs or a big abdomen, follow the shape, if you don't make allowances for this you end up spraying that part of the body closer and it can either build up too much or end up dripping.
the only thing is that you won't get insurance unless you provide proof that you have trained and this is checked by holding a certificate from the training provider.
Can you look at training in other areas maybe?