I said they don't normally need any attention - which is true. I didn't say not to check them.
On the two bikes that I kept for over 100K miles each I replaced a total of three shims, two on the 650 and one on the 900, in each case at around 25K miles.
As the valve stems are hammered during the opening cycle and stretched on closing it seems that the two would more or less cancel out, probably with a bias towards stretching because of the action of the springs. I'd like to see some before and after numbers before accepting that the stretch is measurable.