<font color='#000000'>SELL YOUR HOUSE!! OR TEAR OUT THE ROOF AND CEILING!!
Just kidding!
Going out on a limb here but based on my limited research I'd say don't worry about it. Sure, theoretically a symmetrical room is "ideal" (or at least more predictable) but unless we're talking about a recording studio or speaker testing lab, you're fine. Use the setup program and futz with speaker placement a little and enjoy. If you're feeling techy and/or obsessive-compulsive you can get a spectrum analyzer and parametric EQ unit to make further adjustments!
From what I've read, room effects are most important in the bass, and the assymetry of your room might even help there in breaking up some of the "nodes" that cause boomy bass. Carpeting, drapery and cushy furniture will take care of the treble ricocheting around and smearing the sound.
See the room acoustics page on Siegfried Linkwitz's site, and some of the white papers written by Floyd Toole, available for download at the Infinity Speakers site, for authoritative, yet accessable and common-sense info on room/speaker interactions.
Linkwitz room acoustics page</font>