If you buy a cooler for an overclockable cpu, like a intel "K", then you will get your money's worth.
At least with coolers in the $30-$75 range. The "K" chips are already better binned and ready for overclocking.
On the amd side, the cpu's with the higher clocks are simply better binned and in effect factory overclocked.
I would buy at least a $30 cooler for them if for no other reason but to keep the noise down.