IMO, delaying, because you are actually not doing anything, would be noticable. Some readying, like readying a polearm against a charge, would be noticable. Other readies, like ready to move, is not noticeable. I haven't seen anyone, other than Mavian, posting a ready outside of spoilers. As he said in post 12 of this thread
I like to post my actions the way I do so that they're concise, and easy for the refs to follow if there's a need to go over them.
I haven't seen anything that would indicate the act of readying is non-spoilerable. Most of the time, you are simply delaying your action to a later time, which is as obvious as not taking the action at all. I haven't seen consistent examples of people saying: move action to here, standard action not taken, free actions not taken, done. But if we are to post readied actions and delay as if it's a noticeable thing, than that's fine, but I haven't seen that rule yet (save the proposed post format from today). If that's the ruling than I guess we should decide how far back we go. As I said, my current vote is going back to between my first breath and the crossbow shot. Mavian, would you like to go back further?

About the mount. Mounts use their own actions, so there's no reason they can't ready an action. RAW says guiding a mount with your knees is a free action. It does not list an action value for guiding a mount normally. Since it's not a free, move, or standard action; there's nothing limiting me to guiding it as part of the mounts move action, like it would normally be if it weren't readied. (Not to mention the fact that Kyeudo already acknowledged the mounts readied action.)