Air-conditioners are usually placed at a higher level rather than at a lower level?

hot air rises but not cold air.

EDIT: If you are referring to the fan speed, then a lower setting will allow the air to "condition" properly and blow much cooler air. Often when the fan is set on the highest speed, the air will have less time to cool and the unit may be heating from its own motor, causing the lesser cooled air to warm slightly.

If you are referring to where to place the unit in your home, a higher location will cause the cooler air to circulate better before dropping to a lower level.