I have to agree with Chefster's disagreement.
I don't have any opposition to lines. But, it has been my experience that they offer very little in practical assistance in playing the instrument. There are so many variables that are going on when you stop a note on a fretless fingerboard that the idea that a visual marker is going to help a great deal isn't realistic.
I suppose that the lines might help you at first when playing fairly simply at slow to modest tempos, but if you are really into fretless, you'll be past that before you know it.
Further, while you say that you only want to play quietly at home (when no one else is listening)

, things change. If you develop any proficiency at all on fretless, someone, sometime will want you to drag it out. If you are visually glued to your fb, it makes for a whole bunch of issues.
Again, I'm not knocking a lined fingerboard. I've owned and enjoyed both. I'm just saying that you have an equal opportunity to suck or sound good on either. If I was shopping for a fretless, lines are a non-issue. Although, I do think a no-line, clean look is very cool. And, if I am playing fretless, I want it to be obvious!
Charles