It also depends on what frequency the cut off is omplemented. If at 70Hz, then you are definitely losing something with it on. In which case I would leave it off unless your woofers are pumping hard from rumble.
If it cuts off at 30Hz or below then it may not make much difference leaving it on all the time
If it cuts off at 30Hz or below then it may not make much difference leaving it on all the time