I think its ok to pull your offer prior to getting a response; if so do it as quickly as possible. But its not ok to pull it and lower your offer; not worth the relationship to see how it plays out. Just walk from these at that point. (Only revisit if they sit forever, especially if they come back to you. And then fully explain why you pulled and walked -- you got excited, offered too much, etc but did not want to offend them with a lower offer immediately after pulling your initial one, etc....If they are in fact overpriced and this happens to play out that they sit for this reason, hopefully they will appreciate how you approached them at the time that they thought the higher price was the right price.)
But if they accept before you get a chance to pull your offer, you should honor your offer, IMO.