I'm not extremely proficient with a pick, so I wouldn't say I'm the greatest authority on the matter, but I've never been able to get a sound I like when playing my fretless with a pick. It seems like the percussive attack is ill-conducive to creating that sexy fretless "mwah" that everybody loves. You can certainly play a fretless with a pick, but I think a lot of what makes a fretless "sing" comes from the fingers.