To each his own by all means, but a couple of points to consider: Isolating the earth, of I guess you mean the control contactors, will not stop somebody with ill intent from running the winch.
It is also possible for a control contactor to "weld" closed, or to develop a fault in the hand controller, or its lead, that will prevent you from stopping the winch.
The only "sure" way of making it safe is to have a readily available isolator in the main power lead that cannot be accessed from outside the vehicle. It should be able to be switched from inside the cab and the only way to do that is with a purpose designed and rated soleniod.
Just my 10 cents worth. :)

