Originally Posted By: beanoil
It's been my experience that they tighten, but that's not a rule. Best example I can think of was the original Suzuki/Kawasaki joint effort 250 MX. The valves were dung, and as they wore, they receded into the head, closing the gap. The oiling system on one cam lobe was poor: the head was drilled into the oil valley for one cam lobe, but the other lobe was supposed to survive repeated WFO on splash alone. Drilling the second cam pocket,replacing the worn cam, Kibblewhite Black Diamond valves and leaded fuel helped back then. It's funny how Can-Am wants your valves touched regularly, but they avoid oil changes in a shear machine for 9000 miles.
28k I wouldn't call regular... my two other bikes want 10k and 15k. However I check my DS/dirt bike MUCH more often due to possible issues with valves receding.
Personally I like to check them after a 1-2k miles when new... to get a baseline and then check them periodically to see if their receding or not. Since a lot of new valves are Ti, they rely on a surface treatment to harden them. The harden surface is only microns thick, once that wears... valves start to move quickly (as in possibly needing adjustment with hundreds of miles).