Valve lash can increase or decrease although the general tendency is to decrease.
It depends on where the wear occurs in the valve train; the valve seats tend to wear more than any other part, and that will decrease the valve lash. The cam follower (the bucket in 'shim under bucket') and the cam lobes themselves also wear and that will increase valve lash.
Also as a general rule, cam lobes and followers tend to wear when new and 'seating' and then hold dimension for a long time. Also, with modern finishing techniques and metallurgy, cam lobes and followers wear much less than they did in the good old days.
So everything taken together, valve lash in the systems of C-14 and similar valve arrangements tend to decrease but it is not a rule or the only way they can move.
Brian
That reminds me: It is my understanding that the valve clearance on these bikes will always decrease (or remain constant) with wear. Is that correct, or can the clearance increase with usage?