It's a good idea to let it idle for a short period before shutting off the engine, especially if you have been on the highway recently. For example, you would want to let the car engine idle for about a minute if you were to pull off of the highway at a rest stop. These engines run on boost 100% of the time when you're on the highway, and the turbo is working hot and hard.

However, if you're on your way home, and you drive gently through your neighborhood before pulling into your driveway, there really isn't any need to let the engine idle before shutting it off.

It all depends on how hard you've been working the turbo prior to shutting off the engine.