I asked a buddy about letting the engine run after stopping in my wife's S40 right shorty after we bought it. We've never owned a turbo car before and didn't want to kill it before its time.
He's one of the top Volvo techs in Canada for the last 7 or 8 years.
He laughed and then gave a long technical explanation about rpm, modern oils and machining, oil saturation, heat build-up, etc...
Short of it was he said unless you come screaming into a parking lot or driveway at 6000 rpm and immediately shut the engine off before it has a chance to return to idle, don't worry about it. He stressed heavily the importance of quality oil - he prefers synthetic - and regular oil & filter changes and keeping a close eye on coolant levels.
The best part was when he looked closely at my wife and said "The gauges were put there for a reason, watch them!"
Turbo diesels were something else entirely he said, the low rpm needing more time to cool before shut-down.
Every once in a while I'll take her car out for a blast and when that little 1.9 litre turbo gets moving, it's a LOT of fun. Around 3 grand you can begin to feel it pull and it'll run easily and quickly to 7000 (I've never kept my foot in it past that), pulling hard all the way. But the best part of the car is that the Asian-Warner transmission is perfectly matched to the engine, letting the engine do what it should.
Turbos are a blast!