It doesn't harm it at all. The knife sinking into the pit absorbs the shock, and besides you're about to eat it. Most injuries aren't caused by terrible aim, they're because the pit is slippery and curved, so just a little wrong angle can send the blade sliding off and into the hand. Especially when people whack too hard, and/or started the tequila or cannabis before the guac. That's why you make putting it down a habit, so your muscle memory will override your stupidity.

If you're not going to eat the whole avocado, put it in the fridge with the pit still in it (and the open part covered) to keep it green.