It's true. It doesn't really work like leaving it on full blast. Any potentiometer has a maximum amount of signal that it will let pass, and every pot in a circuit will bleed off some signal. In a guitar circuit, this normally translates to a loss of top end. Removing any controls that you don't need will reduce the amount of signal loss to the minimum possible. It also works with volume controls, but is more noticeable with tone controls. Get rid of the tone control if you don't use it, but don't expect miracles. The difference can be noticeable, but its not huge.