Sure, but it will cause increased wear on the clutch plates. However, idling, keeping the clutch disengaged in neutral for longer periods of time increases wear on the dual-mass flywheel. So, whatever you do, you are damaging something :) . Personally I try to balance it out, but on longer down-hill slopes or when slowly coming to a traffic light I try to keep it in gear. It takes a bit of effort to train, but I really think that you can come close to hybrid economy figures (your mileage may vary :)) ) with thinking while driving. Remember Clarkson from Top Gear driving a 4.0 liter Audi for more than 1200 kilometers with one tank (which is around 82-85 liters) but with a lot of "work" behind the wheel. He got around 6.5 l/100km which is around 36-37 US MPG (or whatever freakin' unit you guys over the pond are using :) ).