I manage a PA31-325 Navajo with the Panther upgrade (so a Lycoming TIO-540 350HP engines instead of 325HP). I've also flown C421s (Continental GITSO-520 with 375hp) & a PA31P (also geared, but a Lycoming GTIO-540 with 425HP).
In all cases I never slam anything open or closed. As a rule of thumb my pax. shouldn't hear much of an obvious change of engine sound in flight from one moment to moment. Descents are done with cruise MP/RPM until 'x' number of minutes prior to arriving in the circuit or at the FAF. 'x' is the difference between cruise MP and the MP appropriate in the circuit or at the FAF. The MP difference, if you like. From the 'x' point on I reduce the MP difference to match the minutes to go to the circuit or FAF. For example, in the Navajo, I need 25" in the circuit/FAF but usually cruise at 33" MP LOP. I descend while maintaining 33" so at 7 minutes prior to ETA I reduce MP to 32", at ETA-6 minutes I reduce to 31", at -5 minutes MP is reduced to 30", and so on. RPM stays at cruise setting. On short final, when the prop is on its low pitch stop I set the RPM levers for max RPM.
I had the owner of the Navajo install a JPI EDM when I took over managing it. As far as I'm concerned an EDM should be standard equipment. Using it I can watch every cylinder. For example, the aircraft's POH specifies to lean at climb power to between 28-32GPH. The CHT on a couple of cylinders rises to an unacceptable level at anything less than 32GPH but that's not shown on the factory gauge. Still within the factory limit 460 or 480 deg F. but damned if I ever want to see CHTs that high. Using the EDM, I've found that 32GPH per engine works well to keep CHTs around 400 in the climb.
Leveling off I leave the cowl flaps open while accelerating, then to half then closed, then set cruise power. I use the CHTs as a guide for this.
Cruise at 65% ROP most CHTs run over 400 deg F, a couple as high as 430. LOP most CHTs are under 400 deg, and only one or two at 400-410. I know I'm comparing equal power production because the TAS is the same. I gain on fuel consumption. I'd rather use excess air (ie LOP) to keep the engine cool instead of excess fuel (ROP).
In descents I can see the CHTs very gradually reduce until by the time I'm in the circuit they're quite cool. I do constant power approaches at reducing speed, adding drag as necessary so there is little in the way of major power changes on the approach and by the time of the last power reduction to flare I'm at threshold speed ie slow.
I taxi in at, or near, idle power which gives the last bit of cooling and lets the turbos spool down still with oil pressure. The couple of minutes taxing in corresponds with an EDM TIT 700. Using that as a cluebat, for short taxi ins I wait at idle for the TIT to reduce to 700 deg. In the same vein, if I have to use a higher power settings to manoeuvre then the cool down period restarts. It's really easy with an EDM - all the CHT & TIT information is right there in front of you.