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.
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!
Wish I could.  It's a multipurpose-room with the audio gear at one end.
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. 
A little late here but yes, a small variation from parallel will benefit you here. If you purposefully make the room asymmetrical to some degree nodes will be far less likely to reinforce or cancel. 
Alcoves of any size are bass collectors so stay away from these.