I have a question: Do fretless P-basses have rosewood fingerboards? If you used rounds, wouldn't it chew up the fingerboard, or is there a coating you put on it?
maple, rosewood, ebony,... those are the most common
but literally any wood can be used for a fretless fingerboard.
Rosewood is most common (I guess that's because of price versus hardness ratio) but ebony or harder is preferred.
And yes, rounds eat up your fingerboard more than flats, but its no big issue.
I use rounds on my fretless, but hey, I wanna groove.
When you're fingerboard is fubar'ed due to rounds, you're probably have been playing that fretless for 20 years and have a thousand gigs under your belt.
The wear is very slow... imediatly noticable but very slow to unplayability.