Chris, no, all channels don't peak simultaneously. This is essentially a non-event in actual movie and music program material, although of course it can take place during laboratory testing with test signals applied to all channels equally. This is why an "all channels driven" power spec is unrealistic for real world home listening and the FTC amplifier power regulations require a two-channels driven basis, explicitly rejecting a proposal to require the all channels standard.
Both listening distance and room size are factors in establishing power requirements, but in typically sized home rooms distance is by far the more significant factor. Studies by Dr. Toole(as reported in his book "Sound Reproduction")and others show that the combined effect of distance and room size lead to a reduction of sound level of about 3dB per doubling of distance, taking into account the contribution of room reflections added to the direct sound. This is sufficiently accurate for estimated power at various sound levels, and I use this for my calculations. For example, a speaker with a typical sensitivity of 89dB at 1 meter with 1 watt would have a comfortably loud 85dB level at a 9-10' listening distance(86dB at 2 meters and 83dB at 4 meters)with that 1 watt, and would use 100 watts at a brief 105dB peak.
Enjoy the music, not the equipment.