My speakers are biampable so it does not matter if the two signals are exactly in phase. One signal goes to the high crossover, one goes to the lower crossover, so there is no risk. I woud never attach the same piece of metal to different outputs.
Down the road I may be using this 7 channel receiver as a stereo receiver, so I am wondering whether the best performance comes from just using speaker mode, or doing what I have described above.