I have thousands of SACDs, mostly in Mch. There is no difference between US, European, Japanese, etc. discs or between channel assignments in processors and players. They never use the back channels for SACD.
The issue appears to be a question of the sound you prefer vs. standards used in recording. Almost all, that is >> 95% of Mch SACDs, BD-A music discs too, are recorded, mixed and mastered assuming the ITU speaker layout with speakers at 0, +- 30 and +- 110 degrees. Feel free to google the ITU 5.0/5.1 layout. I have been to Mch recording studios, seen this in action and discussed this with recording engineers.
There are a few other caveats assumed by ITU. It assumes equidistant, identical monopole speakers. The equidistant requirement is handled by distance correction in DSP in the processor or player if your speakers are not physically equidistant.
The identical requirement may be more difficult, but you can get a close approximation by using similarly voiced speakers from the same manufacturer. Bass management to a sub or subs also helps when smaller surrounds are used. DSP EQ is a great help by voicing all channels similarly, in addition to applying room correction.
The monopole speaker issue might be a problem for some who have bought into side surrounds using the common angled dipole or bipole speakers sold as "surrounds". Those are intended for THX (normally at 90 degrees) and are not ideal for imaging in the ITU setup. They tend to provide a more vague, fuzzy wuzzy sound. I never liked THX as a playback idea, especially not for Mch music, but also for movies.
So, in my 7.1 system, there is never a problem playing all my SACDs with my speakers in ITU configuration. The back channels are silent, of course. The sound is terrific, coming consistently much closer than stereo recordings to the sound at live classical concerts, which I frequently attend. If I wanted to, I could use processing like Dolby PL IIz to provide a synthetic fill via the back channels, but I definitely do not prefer that.
The system also works fine for me with movies, using the back channels only if they are in 7.1. But, my experience with 7.1 tells me it was a waste of money. Relatively few BDs are in 7.1, and my feeling is the back channels do not usually add very much vs. 5.1 playback. My back channels, incidentally, are at +- 150 degrees.
But, if you prefer having surround come at you from the back as opposed to the side surrounds, that is your choice, even though that is not what the recording/editing/mastering engineers intended or heard themselves. You would have to reconnect the side surround outputs to the back speakers. However, in doing so, any DSP distance correction, sub crossovers and/or EQ set up in the processor would be applied to the wrong speakers. It is possible in some PC software, like JRiver which I use for music/movie playback, to reconfigure the channel assignments without these complications. But, I do not need or want that. I want my surround channels at +- 110 degrees as per ITU.