Seems like most of the important points have been made already. Especially for an occasional player (who usually plays fretted), fret lines may be helpful, and they probably won't hurt the sound, and sometimes they even look ok. But personally I think the side lines are a better compromise, assuming you want a little extra help, because to me there's something really special about looking at that big expanse of unmarked ebony (or maybe something else, to each his own).
My fretless isn't quite as challenging as Rami's - only 5 strings, 35 1/2 inches :-), and I went with just the standard side dots. The pure, clean look was a given from the start, but for a while I was considering LEDs instead of dots.
While I was pondering that, I came to realize that my playing was usually best when I wasn't looking at the fingerboard. You really have to listen carefully, and I think that (at least in my case) devoting some fraction of brain capacity to visually aligning fingers to fret lines is too much of a distraction. Even when I do look, I prefer to be focused on the note/scale pattern in general, rather than where exactly I should position a finger.
In fact, having played only fretless for over ten years now, I really have a hard time playing fretted, because there's just too much visual distraction. Plus, it always seems like the damn frets are right where I think my fingers should go. But I suppose that's a pretty extreme case...
Coming from the other side, I have a friend (guitar player, but he has a fretted bass just for fun) who stops over and sometimes tries to play my fretless. After the first couple minutes, he always asks, remind me where I'm supposed to put my fingers - just behind the dot, or right on top?, and the best I can tell him is it depends.
In the first few fret positions (on mine) you have to play a little behind the dot, but halfway up the neck it's more like right over it, and by the time you get up around 17 or so, then you really have to do it by ear. Then again, if I'm playing with a lot of pressure and really into it, sometimes I'll look over and notice that I'm way behind the dot - but the extra pressure, or finger angle, whatever, is handling the intonation just fine.
Anyway, the point is just that the lines may offer some helpful hints, but it's not enough to really depend on, and for some people they can actually be a distraction. So if I were considering lines at all, I'd just put them on the side.
As always, just my opinion.
-Bob
