In reply to Enty: but in accordance to certain carbon steerers you should leave a minimum of 5mm so it reduces the risk of the carbon fraying or breaking.
A maximum stack height of 2cm is allowed below the stem and a single 5mm spacer must always – always – be stacked above. A “slammed down” stack height is preferable; meaning that the stem is positioned directly on the top race of the headset.
I must admit that on my mountain bike I also have 25mm of spacers, though this is purely down to laziness, I've never bothered to cut the steerer in the 4 years since I built the bike. Mountain bikes aren't covered by the rules though.