Broadly speaking, lines on a fretless are pointless - you can't see them when you're playing, can you?
If you can, try and have a go on a fretless with "fret-end markers" like this:
All the comfort blanket of a fretted bass, but to the audience (and the rest of the band) you look like a fretless-playing God.
If you can't find one with the markers already in place, any decent luthier will insert them into the unlined fretless of your choice for very little money.