To accurately and reliably chamfer all of the keys, I set the wheel sanding deck of my electric belt sander to 45 degrees and went for it.
As I've said in previous posts, most woodworking like this (i.e., many pieces of the same or similar part) is 50% planning, 50% having the right tool(s) and 50% production work (yes, it's very much like Man-Bear-Pig). In this case, all of the sharp key foundations get the same chamfering treatment from me. After running through all 21, they each look like the piece in the photo below.
The chamfer is at the end nearest the player. This way, the extra wood that might have shown through on the completed instrument is stripped away - another example of how woodworking is both a subtractive and additive discipline. The end results of these efforts will look very much like the photo below.
Please note that the back end of the sharp top (the right end of the sharp top in the photo above) is aligned with a pencil line I drew before cutting apart all of the keys. If I screwed up and took off a little more chamfer than required (not possible, right?), all of the sharp tops would still align properly when I glue them up.
The final step is some light sanding of the chamfers; then, I can get back to slotting the guide rail ends of the keys.
Postscript: For those of you who might think this is more properly called a bevel (which is what I did on the balance rail before installing the guide pins), observe: Chamfer.