You definitely lose soundstage depth if placed too close to the front wall. Also be careful of reflective surfaces between and behind the speakers. Large TVs, pictures and windows are the worst because of the glass. At a RMAF a few years ago a presenter was getting some fairly harsh, spitty sound until a few of us convinced him to throw a blanket over a mirror that was between the speakers and pull the speakers away from the front wall more. The difference was easily heard by everyone in the room. Some speakers, as you mention, are designed for corners or to be placed close to the front wall. They are probably ported to the front or use the corner to "load" the room. In my experience, that helps the bass be more pronounced (which may or may not be a good thing) but it does hurt the soundstaging.