According to S. Linkwitz here :
Room Acoustics in section D - Loudspeaker and listener placement
"The wall behind the speakers should be diffusive. The rear radiation from a dipole must not be absorbed or it is no longer a dipole. Similarly, the side walls should not absorb sound at the reflection points but diffuse it. A dipole can even be towed in so that the listener sees the radiation null axis in a wall reflection mirror." Some side reflections actually improve the intelligibility of sounds coming from the speakers.
I find this works well. You'll have most problems with placement relative to your front wall in a small room as planars sound best 4 feet or so away from it.