Most of us started practicing with our own cars, family and neighbors. You are asking for trouble immediately if you think you are going to get "practice" on your paying customers vehicles. Bad word of mouth travels so fast. If you make a mistake, and you will, you're going to wreck your reputation.
I offer work for free when I think it will bring me more paying work in the future. For example, I go to law offices and sell a package deal for all the employees as a perq from the boss. The boss gets his/her car done for free. I detailed a local real estate agents garaged car for free because I knew he would show all his rich buddies. And he did, and I gained 4 customers.