PIC is always responsible. If the instructions received from a marshaller conflict with what the PIC thinks is safe, the PIC should stop the aircraft. I have experienced that some marshallers just don't realize that you have chosen to do something other than what they are thinking. As you depart from what they expect to see, they'll get more frantic, and get more in your way. If you simply stop, they'll figure it out, and it will be much more safe for everyone. At the very least, the PIC is much less responsible for something happening, if they have stopped the aircraft because they thought something was unsafe.