It's pretty hard to predict how things will end up sounding in your room. I'd suggest that you set things up first, and if you notice that you are getting a lot of boomy bass or dead spots throughout the room then you might think about bass traps. Although a lot of issues like that can be solved by better sub placement. Check out this brief thread on these topics:
http://www.aperionaudio.com/community/forums/thread/9490.aspx
Jason Hicks
Aperion Audio Guru