Whether you need acoustic treatment will largely depend on the other reflective surfaces in your room (i.e., do you have carpet or hardwood, windows and flat walls or irregular surfaces, drapes, furniture, etc.). The speakers will still spread sound out pretty great, but any acoustic treatment will, to some extent, "deaden" the sound. That might be good in some applications.
Remember - it's like taking aspirin when you have a headache - you don't take the whole bottle, just one or two. A little treatment to reduce echo-y, cold rooms is good. An anechoic chamber = no bueno.
So, yeah, what bridgman said.
