Correct me if I'm wrong, but that property is only used in postLogin and postRegister methods.
What you'd want to do is edit the RedirectIfAuthenticated aka guest middleware, located at app/Http/Middleware/RedirectIfAuthenticated.php and change the handle method to
public function handle($request, Closure $next)
{
if ($this->auth->check())
{
return new RedirectResponse(url('/dashboard'));
}
return $next($request);
}