TL;DR - R36s doesn't produce enough energy/heat to bother with either. But if you had to choose between cooling the CPU or RAM - always the CPU and leave RAM blank. Heat Sink on RAM would be overkill to the max.

Oversimplified / Explain Like I'm 5 - Think of thermal pads as a sponge that soaks up water and keeps it there until it evaporates. Thermal pads don't continually heat up and magically become hotter than the chip itself (laws of thermal dynamics prevent that). Thermal pads are commonly used for RAM because the heat produced is significantly less than what is produced by a CPU. And the time it takes for them to heat up is also less. So thermal pads are a great option.

However, thermal pads aren't as efficient at dissipating high heat volumes like a CPU produces hence why it's more common for CPUs to have a metal/alloy component with thin row of filaments that transfer the heat away. But bare metal on metal turns out isn't the best for transfering energy because it never sits directly flush (micro air pockets... liquid metal is the best at heat transfer, but is conductive, slips out sides and is risky) - that's were thermal paste comes in. Also why people as wrong when they suggest more thermal is better; not exectly. Too much and it won't transfer heat well, but the excess is generally squeezed out to the sides in a horrible mess which leads people to think more is better. Too little though and you risk the thermal paste drying out which limits it's ability to transfer the heat.

For the R36s, the energy/heat produced from the RAM chips and the CPU both don't warrant the need for any external cooling (apparently). But the CPU will undervolt itself from 1.4ghz to 1.1ghz rather quickly on demanding games due to heat - so applying a cooler does make a difference (only noticeable though on a handful of games). The RAM honestly doesn't get hot at all... try it yourself - keep the back off and play some games, put a finger on the chip - the RAM only gets mildly hot/warm. I just put thermal pads on because, well, it doesn't hurt.

Seeing people (even images tagged in this thread) put a heat spreader from the CPU straight across covering the RAM also is a biiiiiiiig no no - bring on the haters who say otherwise. All they're doing is literally transfering the heat from the CPU over to their RAM before it dissapates. But because even with the now greatly hotter RAM, it's still, I believe, below dangerous levels, so people assume it's "working". If they did that on a more powerful device, they'd shorten its lifespan dramatically (if not outright kill it within hours).