No more so than they do at any other time - after all, you are panning across that same set of surround speakers in other modes (Dolby EX, DTS-ES, Pro Logic IIx) in a very similar manner. Heck, in 7 Stereo mode, you are copying the left and right signals from the fronts all the way around, and this is just a scaled-back version of the same thing. The calibration you did on the 990 set the speakers' output levels so they match, and checking the BD50's outputs after splitting the signals will make sure the signals start out level when they enter the 990. As long as you don't have a level mis-match (such as the entire rear soundstage running too low or too high, or the rears higher than the sides) you should be fine.