The question of whether or not Bark Off can stop your dog from barking at all is a rather good one. The natural inclination is to say that if a dog owner uses the tool effectively, that it can condition a dog to stop barking altogether. This may be wishful thinking though. Not all dogs bark, and some bark a lot, but the idea of stopping a dog from barking completely is probably farfetched.
It is likely better to use the tool when it is most needed in order to maintain the effectiveness. There is always the danger that your dog may become de-conditioned to the ultra high frequency and just won’t care any longer. Since the frequencies are not supposed to cause any harm or discomfort for your dog, it is feasible that some dogs may just tune it out and go on barking like before.
The best practice is probably to just use the tool when you have house guests, or when you let your dog out at night and don’t want to wake the neighbors.