I'm in a discussion with a customer that is ordering two custom psalteries. One of his requests is for me to change the notes that I mark with dots. I have always marked the C's on the chromatic psalteries with a single dot. I recently started marking the G's with double dots. His request is to mark the C's and F's because he is used to playing the psaltery he learned on with those notes marked.
He has been playing along with dulcimer players and when he is playing in D he says the C and F naturals being marked tell him which notes to avoid when he's playing along the right side. To me, that thinking is fine if you are only playing in C or D, but may cause confusion when moving to other keys. I've seen builder's mark different notes on their psalteries, but it seems the majority mark only C's, or C's and G's. A few don't mark any notes. I noticed that Song of the Wood marks C's and G's and then also marks C#'s and F#'s. I think you have plenty of hints on the left side because of the pin spacing, two together the first is C# and three together the first is F#, of course the exception there can be the lowest and highest notes on the sharp/flat side where the psaltery may start where you don't have the entire two and three pattern.
I think any series of markings are fine, as long as the player understands it. This could cause some confusion for someone buying a used psaltery with a marking system that is different from what they are used too.