Originally Posted by arnyk
Getting back to the basic question, a lot depends on the details of the room. If you sit relatively close and the room is well-damped you may be more critical of how the speakers and screen are placed with respect to each other. If you have a reverberant room and listen from a longer distance (compared to the ceiling height) then not so much.
In a normal sized listening room with reasonably low reverbation, placing a typical speaker closer to a reflective boundary surface will usually make it easier to locate as a point source. That's how the LEDR stereo-illusion was created in the early 80s. By adding strong artificial reverbation to the sound stimuli, the sound stimuli appeared to move towards the ceiling etc.
In my experience, placing main speakers close to the ceiling in a domestic sized listening room almost always tend to shift the perceived soundstage upward or smear the soundstage to beyond recognition...even if you angle it downward. Only in very reverbant domestic spaces you can get away with this placement, but at the cost of pretty severe soundstage smearing, diffuseness.
Then, of course there are situations where you want a smeared, diffuse soundstage full of reflections and inter-sound time differences, like for the surround channels. In my setup I use 8 surround speakers (four for left and four for right side) placed purposely quite close to the ceiling, at slightly different heights (to create time misaligned mirror sources), and evenly spaced out angularly from an angle slighty in front of the "sweetspot", to almost right behind me. This creates a truly immersive experience of the surround channels which contain mostly ambient sound effects that are not meant to be localized by the ear.
Main speakers near an undamped ceiling in what aspires to be a dedicated theatre room...nah...