The importance of teaching your dog to go both ways round the stock
Just as most humans are left or right-handed, the majority of herding dogs favour working in one direction over another. It's simply a habit that can fairly easily be trained out of the dog.
Unfortunately, some sheep and cattle dog handlers are not bothered if their dog will only go in one direction to gather the stock - as long as it brings them successfully. This video demonstrates the importance of training your dog to work fluently in whichever direction you choose to send it.