If you are using stock rockers and something less than a .500" lift cam, shimming the shafts is a big waste of time, UNLESS you are encountering valve float from pumped up lifters.

I had a small block that after a rebuild with stock rockers and the block and deck milled, would not rev over 4500 rpm. Turned out the lifters were pumping up at that rpm and causing the valves to hang open. UNDER THOSE CIRCUMSTANCES, I would shim the rocker shafts, OR run adjustable rocker arms.

Shimming the shafts is an old NHRA Stocker racer trick back when they were forced to use stock rockers (mostly used on stamped steel, non adjustable rockers and pushrods.) It would help them get a few more RPM out of the motor by lessening the chance the lifters would pump up and cause valve float.