I have two fretless basses. I find the side markers very useful, especially when playing with my band and the lighting is low.
One bass is Hohner PJ Bass F1. It's lined, but they are very very subtle. The side markers are near the fret-line, just slightly past the "fret", slightly sharp. Ive had it forever and learned how to play fretless on it, so when I play it I know to be above the dot.
My other fretless is a Fender MIM Precision that was converted to fretless by a pro. It is lined, but the lines are subtle. Since the bass was originally fretted, the side dots were in the middle of the frets. I carefully painted over the original dots with brown nail polish, and then painted white dots outlined with brown at the frets.
Here's a closeup photo of the Hohner.
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Here's a photo of the P bass and my nail-polish modification.
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Does the white nail polish stay on when accidentally touch it a few times?