I don't use fret markers. As Dusty said, "The diatonic fretboard IS your fret marker." I have never put them on any of the dulcimers I've built either. Doesn't seem to be a problem for anyone.
"The dulcimer sings a sweet song."
My first dulcimer had no fret markers, and I managed fine. The diatonic fretboard IS your fret marker. The first skinny fret is 3. The last fat fret before the three skinnies (assuming you have a 6+) is 5. The last of the three skinnies is 7.
Since then I've bought instruments that have fret markers, and you do get accustomed to them. But unlike the banjo, the dulcimer does not need them.
I think now the main reasons to mark at 3, 5, and 7--even if you don't play only on the melody string--is because that's standard and you want to get used to the way most dulcimers are designed so you won't get confused if you switch dulcimers. And yes, those fret mark stickers look great and come off very easily.
Thanks Susie, Ken and Tukano, I've gone ahead and ordered some stickers. I just hope they're as removable as the vendors say because I'm sure I'll have to reposition at least one of them. The information about why people mark those particular frets was most helpful, Ken, and I thank you for that.
If you really want to have choices for fret markers, I'd suggest this place:
I've used them on occasion for various instruments (for different purposes) over the years. Easy to apply, look really nice, and you can remove them without residue.
When I recently purchased an instrument without a fret marker on a particular fret that I was used to, I added a sticker to the fretboard. Here's that instrument. All but one of the markers are actual inlays, one is a sticker. Can you tell which is which?
Folks use bits of Avery labels for temporary fret markers. There is no standard for marker positions, but as says, markers at frets 3, 7 and 10 are common. Why? Fret 3 is the start of the Ionian modal scale, Fret 7 is the end of the Mixolydian modal scale (which starts on the open fret). Fret 10 is the end of the Ionian modal scale.
On which frets is it customary to have markers? I've not seen enough dulcimers yet to know (ask me about banjo fret markers--THAT I can tell you!) I'm thinking of putting some temporary stickers on my frets so I'll know what the heck is what. Eventually I'll learn where my fingers are, but it would be nice not to have to stop and count all the time! Thanks!