Direction of cutting doesn't make a difference. Condition of the undercoat and length of the cut does.
With that being said, I have never seen a dog have baldness or short patches when being shaved with the grain of the coat. I have only seen this when dogs are shaved in reverse. So how does the direction of cutting not play any role in it?
And on the subject of terriers, I was referring to the terriers owned by pet owners, as most of them have never had their dog's stripped before and won't ever. Therefore they will always be soft and lighter colored. I don't know many terrier people who shave their dogs down to be stripped when it grows back. Depending on the dog, that could take a lot of work.