Safe steerer spacers stack height I'm building my first road bike, which means I'll have to find appropriate bar height through personal experience and cut the fork steerer in process at least once. I plan to start with the following combination, from down to top: 


8mm conical spacer that came with headset.
20mm and 10mm spacers.
40mm high stem.
5mm spacer.


In summary, 38mm before stem, 5 after, resulting in total of 83mm of exposed steerer after cut. The fork has 1 1/8 inch (~28mm) tapered carbon fibre steerer, without metal liner inside. Headset is integrated, upper bearing is almost flush with head tube.

Will it be safe or I'll have to cut shorter? Fork manufacturer did not answer the question. I do not plan to ride out of saddle too much, at least during initial fitting, so bars won't experience significant load, and I'm pretty lightweight.
A total spacer height of 30mm below and 5mm above is quite typical, and certainly nothing to be worried about.  The most important thing, since you have a carbon steer tube, is to make sure you use the proper compression plug and the specified amount of torque when assembling it.  Also make sure the top of your top spacer is just slightly higher than the top of the steer tube, because the top cap needs to rest on the spacer (or stem), not the tube.

Don't forget you can achieve some bar height adjustment by flipping the stem, too.