Depends how near perfection you want to get. Here is an experience from a less technical listener. I have M60s and quiet frankly, I have had a hard time making them sound "bad". We have a relatively new house so as you can imagine, my wife has changed our living room around more times than I can remember and while different placements definitely do sound different, never did they ever sound bad compared to my old gear.
If you are looking for audio bliss, I will agree that the couch on the right will noticeably ruin your 'sweet spot'. In our latest configuration, (thankfully back to the first and best,) there is a single wicker chair beside my right speaker on the inside. It's not directly in front of it but the seat section does extend out into the cone of sound that would eminate from it.
When I don't move the chair, I have to sit further to the right on my centered couch in order to not feel like the right speaker is lacking something. Bugs me enough that I'll move the chair if I'm making a point of sitting and listening to music. If I'm wandering around house or doing anything and the music is background It's not noticeable enough to notice.
It's still 10 times better than I had before so I'm not complaining. Just saying it's all relative to how perfect you want it to be.
