Clearances for steel valves close over time/use regardless of whether they are screw and locknut or shim type due to valve seat wear or recession (and to a small degree the valve's face), making the valve sit deeper in the head. The seats will keep receding until the valve stem is in constant contact with the cam/rocker/tappet (no clearance) and the valve will not close properly with resulting lack of compression, power and possibly burnt valves.

High spec titanium valves can stretch a little over time and could possibly cause the clearances to widen, but this is often counteracted by the wear in the seat, so you don't often notice it happen. Titanium valve equipped bikes are almost always adjusted by shims and have much more regular valve clearance checks.

Ideally, for perfect engine performance, you want them as tight as you dare go without the minimal amount of clearance to allow for thermal expansion. However, in the real world, that would mean adjusting them every 1000 miles or so! The only reason we have 12000 mile valve adjustment intervals on the XT is because the clearances Yamaha decided on are on the sloppy side to accommodate valve seat recession. Theoretically the minimum they quote should be sufficient to last 12000 miles.

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