Depends On The Speaker Type...
The surround back placement isn't going to be as critical as L/C/R and main surrounds. However, if they are direct radiating (not bipole/dipole), it's typically suggested to have them aimed towards the listening position in the same manner as the mains are, regardless of the height.
Bipole/Dipole speakers will be much more forgiving of the placement location, so you may want to experiment with your listening environment to see what's the best location from a soundfield and mounting convenience perspective when using these... -TD