Having owned a GD model up to 112,000 miles and paid full main dealer service throughout ownership since new I now believe despite having been charged for the valve clearances to be adjusted three times they never actually were.
With our current GE (2010) owned since 36,000 and five years old I checked the valve clearances at 40,000 and just this year at 75,000. Minimal if any adjustment necessary (certainly not worth the effort!).
So in conclusion, I agree with Culzean don't bother unless there are other symptoms ie excessive noise or poor fuel consumption etc. These engines are pretty built proof.