Agree....no problem and no strain on the AVR by connecting the back speakers on the Front L & R of the unit.
Only thing to remember is that if you ever want to play back Movie or Music encoded in 5.1, it will have no effect to the back as you are now listening to information from the back that was suppose to come from the front.
eg listening to a live music concert the encoding normally have the ambient sound coming through the surrounds and the musical instruments from the front, as you are viewing this on your TV.
In the format you have it connected, you will only hear the same information for the front, now also coming from the back. So in fact there is no need to play the BD/DVD back in 5.1, you should select PCM stereo on the menu of the disc before starting the video.
Same will apply for movies, so this could be a bit awkward listening too...