1964 KHD Should I Clean?

Well considering it is basically worth bullion value now, and even if you remove that tarnish it will still be worth bullion value you can't really hurt it too much. So the gentlest way I would suggest is dip it in a silver jewelry cleaning dip, rinse VERY THOROUGHLY with tap water (this is very important because ANY traces of dip left on the coin will eventually cause spotting/corrosion), then rinse well with distilled water to flush away any chemicals used to treat your tap water, and finally a flowing rinse with pure acetone to flush away any water on the coin. The acetone will them air dry in a matter of a few seconds and carry off any remaining traces of water leaving you with a clean dry coin.

I don't reccomend doing this, it is just the safest method. If you are just trying to lighten the tarnish you might try diluting the silver dip with distilled water to about 10% dip 90% water. This will slow the reaction down enough than you can watch it and remove the coin before all the toning is gone. Full strength dip will remove it completely before you can dip, remove, and rinse,

Do not clean . Will not benefit the coin .

That toning is pretty tight.