Mike, it is safer to grind them while off of the head. In this way no grindings go onto the other parts. In fact I use my "KwickWay" valve grinding machine to do a precise job on them one at a time. However in the early days I would grind them on a regular bench grinder. Even new ball adjusters have a "flat" top and are not a perfect sphere. Consider the wear pattern and the rocker movement which wears both the ball and the rocker socket over the years. If the valves were very rarely adjusted then a non-symmetrical wear pattern would develop. When the valves finally get adjusted, the new position of the ball adjusters with sockets is uneven and not matched well. The unevenness and dissimilarities of the new mating surfaces can cause issues with the adjustment and/or noise. Regular valve adjustments keep wear patterns more even and minimal. Replacing the ball adjusters or grinding flat the crowns (only if needed), will allow more accurate adjustments and longer lasting adjustments. Realize that replacing all worn parts with new parts is best but also most expensive. Installing new ball adjusters with worn rocker arm sockets, may be another issue, and then if you use new rocker arms, then you have new rocker arms riding on an old camshaft. It can become an expensive never-ending cycle of replacing parts. Some new parts may be NLA from time to time, or different "up-dated" versions of parts can create other issues. Re-conditioning parts to conform to factories specifications and "acceptable wear limits" requires a lot of experience and technical information. Making good judgements can allow good results without replacing everything. Good advice from others with more experience can keep your restoration cost from going out-of-sight.
