I have two bikes with all-carbon Easton forks and have cut both steerers to allow a 10 mm spacer above the stems. One bike has 49,000 miles and the other 43,000 miles with no signs of distress so I have to conclude it hasn't done any harm. I use Profile Designs expansion plugs and these support the stem quite well.
Another important thing is to tighten the stem clamp bolts to the recommended or less torque. I use a torque wrench and usually tighten them to 4 N-m which is a bit less than the recommended maximum. I expect a lot of steerer failures come from insufficient support of the stem and excessive clamp bolt torque.
Last edited by HillRider; 07-01-19 at 06:29 AM.
Reason: Incorrect spacer height given