I am considering an AV system, but the front speakers would serve dual-purpose as stereo speakers too. Has anyone come across problems with the surround speakers microphoning in detriment to the sound when just the front two are in use? If so, is it a simple matter of covering the unused ones with some heavy material?

Jonathan.
I can remember a time ( we were only discussing this at 11 o'clock last night) when even a telephone in the same room as the main speakers was frowned upon.
I sat for hours yesterday listening to a pair of main stero speakers, where not two feet away from each of them were 4 10" bass speakers 2 N Subs were also in the room, and at the rear were a further 8 bass units.
The effect?
Stunning. They did not interfere with my listening one jot.
See the thread 'a close encounter with special 25s'
The front speakers are evidence masters, rears evidence temptations, the stereo pair special 25s.
Then add in the centre & N subs.
I have never sat & listened in a room with that many cones in my entire life.
Dont bother covering the others at all.
Just try & audition in your particular room.

I must admit to being a bit sceptical about the effect myself. However, I did start out in serious hi-fi when it was considered de rigeur for top end hi-fi to be deomonstrated in single-speaker demo rooms and am interested in checking whether this was all overblown.

quote:

interested in checking whether this was all overblown

To a huge extent it most certainly was if yesterday was anything to go by.

I live with compact stand speakers with a pair of rear surrounds and a centre channel + sub in my music room which is also the family living room.
I have yet to be distracted in there by any extraneous resonances.