Above all else, operate the engines as the Flight Manual and engine Operating Manual state. Thereafter, it is most common to reduce throttle, then propeller RPM, rather than RPM then power. If noise abatement requires, reducing from 2700 to 2500 RPM first after takeoff is probably okay - if that is the reason for doing it. Other than that, I would not touch the props until the power was reduced.
Reducing RPM while the power is up, is reducing your detonation margins. I have done detonation testing on several Continentals. I have made these engines detonate - it can be done.
If you make the engine detonate, it will be damaged. You have seconds to undo it (though you won't know it's happening) before pistons start melting. Engine temperatures will affect the detonation margins too. Hotter is worse.
Simply put, detonation takes time and power to happen. You give it the power, by having the throttles open. You give it time by slowing RPM. So the faster the engines are turning, the less time (per engine rotation) as available for detonation to occur, before the next cycle begins, and removes the opportunity anyway. So if you reduce power while leaving RPM faster, you are giving more margin for detonation - good for the engines. Opposite, is not so good.
So, I operate engines that way. If someone tells me different, I ask them to show me in the Flight Manual, but I won't do it differently from that, without a darn good reason. The NIMBY's who want quiet around the airport, 'cause they did not notice it there when they bought the cheap house, don't care about your engines detonating. Even your instructor might not be really worried, as long as the engines keep running for them.
Some Lycomings, and all turbo'd engines I've flown allow some over square, so do what the manual says. The best thing you can do is to understand yourself, and make your own informed decision. So good on you for asking, and welcome to PPRuNe......