Regarding "The data pins will be wired to Pins 6,8,9,12 and 13 and each will have a 470 ohm 1/2w resistor on each pin", note that WS2812's typically are connected together in strings, with the DO (data out) pin of each upstream unit connected to the DI (data in) pin of the next unit downstream. Thus, there is no need to allocate five Arduino IO pins; one is enough.
For further background on WS2812 mechanisms, see for example Is it possible to infer the length of a neopixel string using 1-wire protocol?, and see discussions and diagrams at adafruit.com and sparkfun.com.
Regarding use of an efficient step down switching-supply voltage regulator set for 12 V out, if you use an adjustable regulator you might as well set it lower than that, for example at 8 or 9 volts. Typically, the regulators on Arduino boards that convert Vin to +5 or +3.3 V are linear regulators rather than switching regulators, so every unnecessary extra volt (above the dropout limit) proportionally increases the amount of wasted energy.
In typical use, a WS2812 draws 20 to 60 mA current, supposing up-to-five-volt power. Thus, a string of 30 to 40 of them will draw at least 0.6 A, in most cases, and up to 2.4 A, if all are on at top brightness. In either case, an external power supply would be de rigueur. You would connect the external supply's ground to the Nano's ground; the +5 V's need not be connected together.
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