Not necessarily. I was recently surprised to find out that Cannondale specifically says NOT to use any spacers above their stem. If you click the PDF link I posted above, you will see that they actually recommend cutting the steerer 2-3mm below the top of the stem. However, I think this is because Cannondale requires the use of a special compression cap with an aluminum sleeve that extends into the steerer to reinforce the area where the stem clamps down. If spacers are used above the stem the aluminum sleeve won't be able to reach down far enough to fully support the stem clamp area. IMO it would have been safer and even stronger if they required a 5mm spacer above the stem and simply extended the aluminum sleeve by 5mm. Not sure why they intentionally wanted the steerer below the top of the stem. That goes against what most people consider common practice today.