Check the ceiling
There are few rules in D&D that have no exceptions. Every strategy has advantages and disadvantages. The axioms in this book are strategies which are on average a net positive for dungeon effectiveness. Not so with checking the ceiling. It’s all upside. I suppose one day I’ll run into a dungeon where every archway has an Explosive Rune carved into its apex, but until that day I’m going to claim that you should always, always check the ceiling.
This kind of action, like probing ahead with a 10’ pole, looking through keyholes, listening at doors, falls into a category I like to call “exploration doctrine”. The set of actions you take when exploring a door, room, or corridor, to safely detect dangers before they kill you. There’s a trade-off between thorough examination of the environment, and time spent (incurring the risk of random encounters). Optimal exploration doctrine considers the kinds of threats previously encountered (are there lots of traps? How deadly are they?) and the nature of random encounters (can your party handily defeat any likely encounters?).
In practice I find that there’s little to be gained from micromanaging exploration. It gets tedious listing every single action you take every single time, for fear of the DM yelling “Gotcha! You didn’t say you checked the ceiling this time!” I recommend an explicit and fairly static standard exploration doctrine. Write down your door procedure, your corridor procedure and so on. List each action you take, and how long the whole procedure takes. Tell the DM you’re using this standard approach unless you explicitly say otherwise. It’ll save time in play, it saves pointless bickering about who did what when, and it ensures that you always, always check the ceiling.
