2 questions

[Peter Som]

1) Hitting while moving backyards can be a tricky ordeal. This question can be put into maaaaaaaaaaaany different contexts, thus very hard to answer. The only punch that you can reliably throw while moving is the jab. That's why you hear every boxing trainer and boxer talking about the importance of the jab. Literally every other punch requires you to set your feet to throw the punch. If you tried to throw a power punch while on the move, you'll quickly learn that your balance is completely compromised.

With this in mind, you wont typically throw punches while moving backyards unless it's a jab. There has to be a reason why you're only moving backwards (I'm assuming you're only moving backwards...I'm also assuming you're also moving laterally to not let your opponent set his/her feet). Otherwise, there aren't many benefits to move backwards unless you're hurt, in which case you would shoot your jab to smother any kind of offense your opponent is trying to mount.

There are very few and rare circumstances where you would get a counter off someone who is walking you down. One of them is that they get over excited and actually run to you instead of properly pushing off his back foot and glide to you, in which you can confidently put a stiff jab to his nose, or set your feet to throw a right cross or a properly timed left hook. Another scenario is you catch them with a punch moving back and pivoting. And the last scenario I can think of is not necessarily moving back all of the way, but timing a punch off of moving forward with a punch. They're expecting you to keep moving backwards, time your steps so that you step less and less, so that when the moment is right, you step forward just enough to be in range, and clock him with a well timed punch.

So in summary to your first question, the only punch you can realistically throw while moving is your jab, and only moving backwards is a very primitive and often less fruitful tactic.

2) Good posture is everything. You will not get maximum leverage on your punches without them. You can't defend properly without good posture either. There are no cons with having good posture, develop good posture, because not having good posture is having bad posture . That is all.