One of the problems with this discussion is the question seems to be binary
Yes shock cooling is a big deal or No it is nothing to worry about.
My personal opinion is for the simple 4 cylinder engines fitted to your typical club trainer/tourer it is pretty much a non issue. The flying school I help out at has permission to go to 3500 hours before overhauls on the C 172 engines (0320 D2J with 2000 hr manufacturers recommended TBO). They live a hard life with lots of idle power approaches frequent power changes in the circuit etc etc and in general no procedures to prevent shock cooling) Most engines make it to 3500 hours with no cylinder work at all.
However I also fly a Cessna C 421 with the 375 hp Geared turbocharged Continental GTSIO 520 engine. The engine manual cautions going abruptly from cruise power to idle power in flight will reduce the engine life by 50 hours
These I think represent the extremes. I think you can divide engines into 4 broad groups
1) Simple 4 cylinder less than 200 hp where precautions to prevent shock cooling while nice to practice are not required.
2) Non turbocharged 6 cylinder more then 200 hp engines where prevention of shock cooling is desirable and will likely extend cylinder life
3) Turbocharged engines making up to 350 Hp where prevention of shock cooling will definitely extend cylinder life and is essential to make TBO
4) High power turbocharged engines of 350 hp or more where prevention of shock cooling is absolutely essential to prevent cylinder damage from occurring over quite short time frames and the engine has no hope of making TBO if regularly shock cooled.
However even for the simple engines my philosophy is to be nice to the thing that is keeping me in the air. So I let it warn up before applying power, watch the oil temp and CHT (if fitted) and especially on warm days, avoid high power low airspeed operations to avoid overheating and try to make all power changes slowly and smoothly.