In short, the answer to your question is "No". I don't hear any sibilance.
You can see how acoustically bright my room is below. I can sit as much as 60 degrees off-axis (in the cushy chair) with no loss of understandability during music or movies. I do however agree that timber changes somewhere around 45 degrees.
It's been said around here many times before that many recordings are engineered with their high frequencies boosted to satisfy people like my wife who listen to music on their boomboxes. If you have recordings like that, use the treble control to tame the highs.
