Hello Marcus,
I would say that is it a matter of personal taste, but ideally you want to reproduce the effect you get in a church with reverb. I usually say to my clients... "turn up the rear speakers volume until you can detect that the sound is coming from the rear speakers then lower it a bit until this effect disappears.
That way you won't detect that sound is coming from the back but it will fill the space and remove the "in your face" sound when using only front speakers. To check the differences once you set your rear speakers correctly, simply mute the rear, you should find that you aren't "as surrounded" by sound as when the rear speakers are on.