For me personally, I find lines plus dots to be too busy and confusing. My ideal fretless has easily visible side dots only on places where there are normal position markers, but the side dots should be on the fret lines not between them. I don't much care if it's lined or not, since that only matters when you're playing sitting down or hunched over the bass. I'd rather trust my ear than my eyes. Using your ear makes you a better musician in the long run.
That said, I have no experience with a fretless Dingwall, and I do have a suspicion I'd be lost without lines simply because I am only a part-time Dingwall player. I think it's fairly likely that if I had trained my muscle memory strictly for fanned frets by playing Dingwalls exclusively, I would probably be fine with it unlined. But that is pure conjecture, since I have never tried it, so it's not even worth the paper it's printed on. If I were buying a used one, I would buy it whether lined or unlined. If I were ordering a new one, I'd order it lined, and would request that the lines be made of a material that has moisture properties as similar to the fingerboard as possible if I wanted to minimize the risk.
Three more things to consider:
1. Adding epoxy coating certainly overcomes the concern about "hearing the fretlines" when sliding. However, it also drastically changes the tonality of the bass, making it much brighter and crisper, adding a slight degree of raspiness to the tone if you have the action super low for maximum mwah. So you would need to want that type of tone, or you won't be happy. I have one such fretless, so I know this from experience.
2. If you have an untreated fingerboard, you really should be keeping it fully hydrated with lemon oil anyway so it doesn't dry out. And if it's fully hydrated with oil, then there is no room for moisture to get into it. Therefore, it is going to be very stable, and won't appreciably expand or contract anyway, thus making your concern about "hearing the fretlines" a non-issue.
3. Even if you had an untreated fb and you failed to keep it hydrated, and the fingerboard shrunk from dryness faster than the fretline material shrunk, so the fretlines end up protruding a wee bit, then how closely would you (or your audience) need to be paying attention in order to actually be able to "hear the fretlines"? If this is a high quality, very hifi solo recording with bright settings (like Damian), then OK, maybe you could hear them. Maybe. But again, if this happens, it's because you failed to keep the fb properly hydrated with lemon oil. Go back to item 2.
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