Just like it's fretted counterpart, a fretless needs to be correctly tuned. If the saddles are not in the right position, tuning will be out as you move higher up the neck. Obviously you can compensate for this on a fretless, but it will make it much harder to play in tune, if the intonation is not where you expect it to be. Check the harmonic at the 12th fret position against the fingered ('fretted') note. These should both be in tune.
When you play a note on a fretless with your finger perpendicular to the board, you can go from slightly flat to sharp by slightly rolling your finger back and forth, if that makes sense.