[codersteve]

>>I routinely KNOW I have hit an incorrect key, hit backspace, and correct it and progress with the word

Because that's a bad habit when deliberately practicing. A good typing program should have a couple of modes. One that forces you to keep moving forward even if you make a mistake (and not let you backspace, this develops speed). Another one that will make you stop until you hit the correct key (for beginners or to break bad habits).

Depends on what skill you want to focus on.