I've seen this picture as a guideline for setting up a 5.1 system. You are using 5. 1 right? not 7.1?
I'd put them in the corners near or at the angle suggested. My room dimensions are abnormal, so I had to compromise with my 7.1 setup. But if your room allows for it, I'd use the recommended speaker placements and test it.
If you don't like it after that, move them to where you think they sound good to you.