It is a very good idea to check the installed height of the valves and fit the correct shim-otherwise the lifter may be forced to the end of its adjustment capability--resulting in either noisy valves (clearance on valve closed), or a valve not closing all the way or too late in the cycle. Either way its not good. When the heads were 'milled', its extremely important the cam towers be shimmed to the correct height individually--unless both sides of the head were milled and the head surfaces are true parallel. (unlikely since valve guides and casting plugs have to be removed and reinstalled, and head thickness carefully measured all around to ensure against non-parallel finished surfaces. If this cam tower shimming is not done properly, the cam is under bending load because the bearing centers are not the same height, and the cam will fail.