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.