This is a long read (sorry if you've seen this before) but this might help in regards to speaker placement. It just about covers everything about 5.1 mixing...I found it very interesting...

Directly from the recording sound engineers...
Producers and Engineers

Dana

M80's, QS8's, M22's, CHT SHO-10, Dual CHT SS-18.1's, Onkyo NR3008, Mitsubishi WD-73740