Oyyyyy.... "square, over square, under square"..... even "throttle first"... there is NO rule or rule of thumb that covers all applications. First off, the whole "over square" thing (RPM in 1000s lower than MP in inches) ruining your engines is a crock of crap. Minimum RPM with maximum manifold pressure will give you greatest range and efficiency. I'll refer you to the Lindbergh Doctrine from pre WWII when Lindbergh was showing Chenaults men in Burma how to increase their combat patrol and attack mission range in the P-40s. You do need to watch out for some RPM limitations due to harmonic issues, but they are case specific and placarded for applicable engine/prop combinations.
Throttle First..... Can't think of a better rule to kill a GTSIO 520. All my power reductions from altitude are done props first, and depending on the engines and mission, I may never touch the throttles from the point where I advance them to full for takeoff until I'm about to land, doing all my power adjustments with props, mixture, and potentially wastegates.