Actually the many vent holes on the top of the 722K (and some other DishPro boxes: 50X's, 301, 311, 322) are sufficient to allow the heat to rise out of the box, provided you don't put something on top that covers the holes and prevents the heat from escaping.
The problem boxes with heat are the ViP DVR models that are solid on top and a few side slots and vents on the bottom (where the fan is located) that don't allow for as much heat to escape. If those units are in tight quarters, then a heat build-up problem can occur and additional solutions can make a big difference. I use casters on the feet to allow heat to escape and air to flow through the bottom, and anything placed on top of the box is also on casters so that the heat can escape the from the solid top of the DVR and not heat up the unit on top of it, and it works great.
I really don't think you need the fan for the 222K. The normal operating temp is over 100 degrees anyway, and it should be performing well, for a 722K, that is. It would be nice of the ViP DVR's had all those holes on top; then we wouldn't need all the casters.