Optimal vertical position of bass traps in corner?

Assuming I have a pair of 4ft tall bass traps, what is the ideal vertical location to place these in the corners? I notice that most people just place the bass traps on the floor but I was wondering if it perhaps would be better to put them in a different location like centered along the vertical or closer to the ceiling, etc. Does vertical placement of bass traps even make a difference?
Yes, it does. Placing it in a tricorner (intersection of three boundaries) will allow it to interact maximally with all three major room modes (L,W,H). This means that placing it in the corner at the floor or at the ceiling is more effective than at mid-wall.
Kr is right. However the interplay of sound pressure in a room is complex. I would experiment. Couches and chairs + decor have can be used to good effect on the ground. Try upper front corners to start. Enjoy the process. It will make you a better listener and then give more enjoyment to your listening.