should be on floor.
Ceiling vents should be directed towards the room to evenly distribute air throughout the space. Pointing them towards the window would not effectively cool or heat the room.
on the floor on the floor
Heating vents should be located near the floor since hot air rises. If the heating vents are located near the ceiling the heat would stay there and not warm the room.
Ceiling. Cold air fails
Vents should be situated near windows. Windows that are north or west facing should get a vent before a south or east facing window would get one. (Assuming that the vents also supply heat.)
If your air and heating vents are blowing from the ceiling, your ceiling fan should blow air downward to help circulate the air in the room. This can help distribute the heated or cooled air more efficiently and create a more comfortable environment.
up in the summer and down in the winter
Cool air falls toward the floor. Deflectors shouldn't be necessary unless the vent is behind a curtain or something, or if you want to deflect the cool air toward your easy chair.
Heating vents are typically placed near the floor to ensure better air circulation and distribution of warm air throughout the room. Placing them near the ceiling may lead to stratification of air, where the warm air stays near the ceiling and doesn't effectively heat the room.
I believe the upper vents are for A/C only. The floor vents are for the Hot air. Test your ceiling controls by moving the knob from cold to hot, you should hear the air pass from upper vents to lower vents. Providing of course you have selected REAR from the front control panel.
When you say "exhaust vents in your ceiling" I assume you are referriong to the heating and AC vents inside your house and not the roof vent on the outside used to vent the attic. It is most likely that the duct work was ran in the ceiling for ease of installtion or the house may be built on a slab and they did not want to run the duct work through the slab.