non parallel walls in listening room


I have the chance to build a listening room about 16' x 30'.  I'll be setting up my 2 channel along the short side and listening towards the middle/back.  I can make the 30' wall non-parallel.  My question is exactly non-parallel does one need to reduce resonances?  Are we talking a degree or so, or a few inches along the whole length, or a couple feet??  Anything more than a few inches would be hard for me to do and I wonder if a small amount would make a difference.

Any thoughts would be greatly appreciated.
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IMO your missing an opportunity here to remove the room from the equation.By setting up in the center of the long wall you will eliminate early reflections,echo & bass nodes.Treat all corners,high & low.Split the long wall behind electronics(keep as low as possible) into 3 sections & treat with 2'x4' panels.Echo treatment centered over listening area @ ceiling junction & over electronics.2 small panels on wall behind listening chair/s.Keep subs off the floor using Auralex SubDude's & your room will sing!
It's not just about resonances. Perfect symmetry for walls is vital for balanced reflections. Professional spaces use non parallel walls and a rising ceiling.