While some people might interpret this as "topping from the bottom," and not "real" submission, scenes with a service top are generally negotiated and agreed to ahead of time while "topping from the bottom" usually comes as a surprise to the top mid-scene.
While in a scene, it's always the responsibility of the top to monitor the bottom's condition, whether or not there is an established power exchange relationship or not. The same safety rules apply to any scene, whether it's a typical Dom/sub relation or in a servicing situation.
Professional doms are the perfect example of service tops. They are paid to provide a topping service to clients.