Yesterday while I had the valve cover off and was measuring valve
clearance, I attempted to compress a valve with a flat-blade
screwdriver to test how easy it might be to remove the shims when the
time comes.

I did not want to scratch anything, so I was being very gentle. I
wasn't able to compress the valve doing this.

How much force is usually required to compress the valve in order to
remove the shim?  Should I be using a large flat blade screwdriver as a
lever, wedged under the cam to push it down? From the Haynes manual it
appeared that all I needed to do was gently push down on the valve with
the screwdriver -- but apparantly this is not the case.

Thanks.
JB