About applying paste on the underside of the CPU....well...I'm not sure. The paste is there to help with thermal conductivity, i.e. heat-transfer from the CPU's heatsink and the main heatsink/fan assembly of the CPU cooler. True, the underside of a CPU can get hot as well, but NOT as hot as the top. Another thing to think about is that many types of cooling-goop contains minute metal/silver particles. Which are electrically conductive. Which will then be situated directly onto the pins of the CPU, which MAY just cause a short, which agan can fry your CPU. I would be extremely careful with doing this. The fact that there is no real reason to have heat traveling from the underside of the CPU and down onto the CPU slot and/or motherboard since there is no cooling to those areas, also makes me curious as to why the MoBo-producer says that you should put goop on the socket/underside of the CPU as well.
There is a great debate about whether or not to have paste/goop on the underside of the CPU. True, it is needed if you are going for a Ultra-high-end Vapochill LS-system (Damn nice, but both noisy and hideously expensive) just since the LS won't allow the system to start before the CPU is pre-chilled to -10c, but that's about it.
To sum it up: There's no real reason to do it, but if you do, follow Fudd's advice: Be VERY careful.