You guys are getting too crazy with this. I have to lean towards agreement with Sheep on this one; for most common configurations/placements of consumer electronics (AVR's, etc.) there should be no need to further establish cooling mechanisms, unless there the thing is tightly packed into a non-ventilated cabinet, or a closet, or elsewhere where there is restricted airflow - this is where you run the risk of something getting too hot to the point where damage will occur. You would be surprised at just how robust most electronics are with respect to heat - consider the stereo in your car and the extreme temperatures it endures, yet it still keeps rocking. Even many digital circuits can withstand temperatures up to 120 degrees before any issue begins to develop, and even then it is only an upper threshold with conservative margins established.
Building special cabinets with high-powered components that are likely to generate a large amount of heat would require extra cooling fan(s). This is the case in many a commercial A/V build I've done, or the larger more complex equipment racks I've built and worked with in TV stations.
Bottom line - heat rises. As long as there is nothing obstructing the vents and there is adequate space to allow air to move up and out, and in from the bottom as it will naturally do, you should have no need for a fan.
