It depends on the specific RAM. My Corsair Dominator for example is rated at DDR3-1600 8-8-8-24 1.65V, but it is also programed to run at DDR3-1333 9-10-10-25 1.5V, DDR3-1186 8-9-9-22 1.5V, and DDR3-888 6-6-6-16 1.5V. All of those could be considered "defaults" for this RAM.
So is it better to purchase a 1.5 1333 memory when considering overclocking or the 1.65
It would seem that a 1.5 chipset stable at 1.5 volts is a better candidate than a 1333 1.65 chip.