A role reversal occurs, when a camgirl asks you to buy her a christmas present. She is a service provider, you are a customer. She should be at your service; not the other way around.
I do advise to tip camgirls only in exchange for your personal satisfication/gratification. I think that you can end up being frustrated if you spend money on camgirls without getting anything in exchange.
For your satisfaction, always condition the money given to a webcam model on a specific service being provided.
