I find that lines throw me off. With a lined bass you have to finger the note you want on the line, not inbetween the lines (frets) like you would on a fretted. Your natural inclination is to do as you learned.
With an unlined, there are dots on the side of the neck and those point to where an actual note is on the fretboard. If you have been playing for a while, IMO, you will find that your intonation will be close if not dead on, like typing on a keyboard without looking at the keys.
Of course, when you are playing with other musicians you have to listen closely to ensure your entonation is where it needs to be. IMO, the beauty of playing fretless is that your selection of notes is exponentially increased because you will find that there are notes in between notes as we know them, if that makes any sense. Ultimately, try out some lined and unlined models and find what you are comfortable with because I think we can all agree that what is comfortable for the rest of us won't necessarily be the same for you.
