Not too crucial where you locate the outlet as far as operational effectiveness is concerned - the PIV unit is gently and continuously pumping fresh air into your house so anywhere convenient on the landing ceiling will be OK. However, you want to keep it away from any smoke alarms - you don't want the air flow forcing smoke away from the sensor!(Nuaire recommend 1 metre for the Drimaster, but you may be able to get away with less, depending on the fan speed set on the unit). Also, bear in mind, if the outlet is close to a wall or above a door, it can send a draught downwards (again, depending on fan speed and distance) which might cause a nuisance. The Drimaster outlet blows air out in 4 directions, across the ceiling. Up to 2 sides can be blanked off (foam inserts supplied) to overcome the smoke alarm/wall issues to some extent. Ultimately the further the outlet is away from walls, the less chance of nuisance draughts.
(Note: I fit a lot of these in my line of work, so am commenting from personal experience of approx. 100 installations in all manner of property types. Shove 'em anywhere - they work!! Hope this helps)