A double bass doesn't have lines.
I play both DB and fretless BG. There's a difference with the DB: The straight portion of the neck spans all of about seven notes. Outside of that span, you have all sorts of "cheats" such as where your hand touches the curve of the neck, your arm touching the upper bout, and so forth. But even on the DB, my own experience is that intonation is not an all-or-nothing affair, but a lifelong pursuit to keep improving. Near the end of his life, a reporter asked Ray Brown if he still needed to practice, and Brown said: Of course, I could still improve my intonation. That's from a bassist who I've never heard play out of tune.
So, following that great master, I think it's no shame to admit that I still need to improve my intonation, and that I will take any help that I can get to play in tune.
Both of my electric basses have lines. The simple reason is that I de-fretted them myself: A 1970s Fender Mustang Bass, and a more recent entry level Ibanez. In each case I filled the slots with a dark colored Plastic Wood. Incidentally, the Plastic Wood has held up for 30+ years on the Fender, and more than a decade on the Ibby. I'd use it again. The color makes the lines nearly invisible from a distance, but I know they're there. I think that I'd prefer lines if I bought a new fretless today. (I'm looking, maybe if I survive the COVID outbreak, I'll reward myself with a new bass. My last new bass was a reward for surviving the 2008 financial crisis).
Now, about using the lines. I think you have to practice and play like they're not there, and only use them for infrequent reference. The reason is simply that there's too much happening for you to pay constant attention to your fingerboard, plus it looks like you're staring at your shoes. Whether anybody can get to the point of playing fretless with good enough intonation is a toss-up, and might not really even reflect how good you are on bass. It's not a matter of ear -- if you can hear that you're out of tune, it's too late. It's mostly technique. It might be more of a matter of how you devote your practice time. Having been around a lot of kids learning string instruments, I've noticed that intonation begins to happen when it becomes almost an obsession. I use intonation as a gauge of my technique, i.e., when I notice that I'm consistently playing something out of tune, I look for what I'm actually doing wrong with my technique, and try to correct it.
There, enough words without answering the question!
