grounding heatsink plate?

I'm asking particularly for the SBL23xx, but I guess I guess this same question would apply for other controllers. I see that the large metal plate on back of the controller, that acts as a big heatsink, is not connected to the controller GND. I was wondering if it's ok to connect it to the controller/system GND? My planned system will use three of the controllers, and I'd like to mount them all to one large metal plate that will act as a big common ground plane for the whole system (that common ground plane is the battery negative terminal). Would this be ok?