Jetboy, I've been tracking mine since the 10,000km mark, and they didn't change by more than 0.03mm in another 12,000km. Curiously, the gaps are increasing.
When I changed the cam chain at 25,000km, I measured each shim, and checked the clearances. 3 exhaust valves were at the top of the range.
At 30,000km, I'll check them again, and any outside the range will get new shims. A painful job, 'cos the cams have to come off to do it.
So I guess what I'm saying is, from my experience, checking every 10,000km is often enough.