A role reversal occurs, when a cam model 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 cam models only in exchange for your personal satisfication/gratification. I think that you can end up being frustrated if you spend money on cam models without getting anything in exchange.
For your satisfaction, always condition the money given to a webcam model on a specific service being provided.
