I used to do the Cancellara thing but due to lack of education/knowledge/pictures I held the bars upside down (like doing a curl). I burned my chin on the front tire on one descent. I eventually learned the proper position. It's possible to use the aero tuck even on short straights - I think it takes less than half a second to swap positions. I tried this on various curvy descents to verify that it didn't take long to move my hands. Rough roads make it less safe to use the aero position.