I do fly a 172 and I realize that the manufacturer's recs take precedence over the book (Airplane Flying Handbook) that I referenced to earlier. I thought it was "interesting" that they mentioned, how it was not a proven fact that holding of brakes reduces the total take off roll. But like one poster mentioned, it is better to observe all the engine instruments whilst applied full power, and that if there was a problem, the aborted take off would be less messier.