You can mount the front speakers on the wall to either side of the TV or on the side walls without seriously affecting the quality of the sound. I always try to keep the fronts somewhere near the same height above the floor as the center,but I have seen them on the floor, in the corners of the wall near the ceiling and just about anywhere else one could mount them. I even put ceiling speakers pointing down in for customers and they sound pretty good.
I would tip speakers down 10-15 degrees depending on how high you mount them. Again, I do not think it will make much difference. It just looks like it would!!
Rear surrounds can also go pretty high on the wall, or even in the ceiling as well. It does not seem to make a lot of difference most of the time.
Speakers are pretty wide in their dispersion characteristics and will sound good at wide angles from their center.
I recommend you just mount them temporarily on a ladder, etc,and give it a try. Then you have dynamically tested the position and can really make a good judgement as to how it affects the listening.
Hope this gives you food for thought. If so, and you have more questions, just come back in with a follow up.