In regards to parrying the jab with the lead hand (matched pair) I always teach, and was taught, that you want to avoid that. As was mentioned earlier, you want to make sure to have your counter ready to fire off in response, in this case, a jab, and more importantly, you want to avoid opening up the path for the right hand. Parrying/blocking the jab with the lead hand places you in line for the right hand. It falls under fundamental miscalculation of attack values/exchanges. Parrying the jab with the lead hand shows too much respect to the jab, and its low risk/low reward nature. It skews the reward wildly in its favor by opening up the line for the right hand.
That's not to say this cannot be used to draw that right hand out, you can circle right and parry the jab with lead hand OUTSIDE the jab and open the path for your own right hand minor counter, but that requires really fast reflexes/feet.
catch or parry the orthodox jab with the trailing (right) hand. But again, that's just the science, as always, there is no must in art...