As usual, they never tell you everything! Just saying 6", 8" on sides or whatever is not enough, could even mislead... One can have 8" on all sides, but in an area where the A/C could not get to, such as far from any air vent registers, or rooms with no AC with the room temperature around 28 deg C, whereas another person could have only 6" on all sides but the ambient temperature around the unit is 23 deg C because the room is cooled by AC, with air registers nearby. Of course the other important factor is, as we all know, the output demand on the AVR.
Manufacturers recommendations are just basic general guidelines, users have to be smart enough to do their own assessments. The easiest and obvious way is to measure the temperature around the unit, on top, sides of the enclosure, and go from there. If in doubt, use an external fan, because the internal fans rarely turn on, it seems D+M included enough fans to minimize their 3 year warranty exposure but set the trigger point high enough to avoid customer complains on noise. Others, such as Anthem, went the opposite way, probably because they do care about longevity, not just to survive the warranty period, and yet their customers offer complain about fan noise.