Get all the speakers to be symmetrical, so to speak.
Then when it's movie time, slide the couch over so that it's centered. No one is going to like having a surround speaker blasting directly into their ear.
Aim for 38% room length from either front or back wall to your ears. That number, which is 6.84', might drop a tad, depending on the room. Yes, lengthwise is definitely best.
If say your eyes are 6' from front wall, and the TV is 1' from front wall (eyes are 5' from TV), and the TV is 42" (can't tell really), your viewing angle is then 33.9 degrees, which is a very good starting point for immersion with 1080p. (THX recommends 36).