I went over the Best Buy meantime and asked the Magnolia room person the same. He said it’s best to balance the room with B&W 704 or 705. He was against using KEFs or Polks or Martin Logans in the back when 702s were in front. He said that they perform quite differently. They sell KEFs and MLs also.
Of course he’s gonna say that because he wants to sell you more speakers and that’s the easy layup answer. I worked at Magnolia briefly and have been to several others, and they are mostly just hacks who have very little personal or real experience in high-end audio. When I cobbled together my HT system I used my wife’s Polk monitors from the 90s that were not in any way matched to my fronts and they worked perfectly fine for creating a very immersive movie experience. If you wanna blow a big wad on the rears have at it, but my opinion and experience is that it is not necessary. Save the money there and put the rest toward getting a better (or second) sub or center speaker where it’ll make a much bigger difference.