If you're spending a lot of money on good monitors - you're better off actually spending some of those funds on room treatment, and getting less expensive monitors.
When monitoring you want as clean a representation of your audio coming out of the speakers. By not having a room that is treated in some way, and depending on the size of the room and position, you will get a lot of cancellation or build up of frequencies where you sit.
I would suggest looking into Auralex Roominator Kits, they are very affordable, and you can use push pins to pin the foam to the wall and if you go with the deluxe set, the diffusers to the walls / ceiling. I have used this a lot in my home studio which has moved quite often between different rental properties.
http://www.auralex.com/ikc-new/ - room size calculator with recommendations for different kits.
I bought the deluxe plus kit which allows me to size up or down depending on the room. Then again I had a reasonable budget and was doing a lot of audio work which required a good mix.
You can get smaller kits for $199-$399
one such example from one reseller:
http://www.sweetwater.com/store/detail/RoomDSTAlBurBass traps are pretty important, as well as the absorbtion of high frequencies. Placing furniture around like bookshelves and sofas .. etc can help scatter the mid frequencies effectively.
If you get a small room kit you can purchase additional elements to continue to build up your room treatment material.
Treating your room so you can hear the stereo field correctly, not have interfereing standing waves does affect your mix. You hear differently in 3D space than two separate speakers pumping sound directly into your ears.