I would always sell at an ATH.
Sure, I believe that should be the case of all traders and regardless of where you buy in but even when we know there would be another ATH this is usually not that easy to point out however, for me I will usually rather have a set percentage gain I want to make in all my trade and I also usually try to buy in at a comfortable low position where I believe that market is about to make a change but this is also not that easy to point.