One of my friends started out as an upright bassist so he is very accustomed to an unlined fingerboard. When he had his fretless bass custom-built it originally had an unlined fingerboard. His fretless bass has a very straight and uniform neck without much taper, so relying upon feel (as I can do with my fretless P-J) did not work for him. This was no problem when he played backup in sessions but he encountered difficulty performing live as a lead singer and frontman. He had lines retrofitted to his bass so that, when necessary, he can flick a very quick glance at the fingerboard to help check his hand position.
He was as surprised by the need for lines as I was that he had them added.

He was as surprised by the need for lines as I was that he had them added.

in another topic), and made it "lineless" as well as fretless.