Congrats on finishing the new room. IME, you have to try orienting speakers both ways to know which will sound better. Going lengthwise I agree that you want the side walls as equal as possible to get symmetric soundstage illusion.
Slanted ceiling angles are usually obtuse, which will not aim corner midrange reflections toward you like right angle corners do. So you might find that the 90 degree corners more valuable for treatment than the 135 degree ceiling angles. Since the slanted ceiling walls are different heights, to achieve soundstage symmetry when shooting lengthwise, place speakers more toward the taller sidewall so the distance from tweeter to ceiling is more equal. Speaker placement has as much effect as treatment, but it takes a long time to do a good job.
Often the most valuable place for bass absorption in a small room is right next to the speakers, leaning against the wall on their side, starting at the front baffle and coming out into the room from there. Covering the floor/wall seam in front of speaker.
Don't blindly follow any rules until you hear and compare for yourself. Sidewall and ceiling reflections as well as front wall SBIR absorbtion are common rules, but they have side effects, especially in lively room. Try it and see. Have fun Andy!
