Crystal Oscillator Accuracy

Well, the build that'd linked above:

• Uses the 8Mhz oscillator normally
  
• In deep sleep, uses the 32kHz osc (it can't use anything else)
  
  

So you get the best of both worlds. It means it'll be accurate normally, but if you use setDeepSleep it won't be as good. I think it's a reasonable compromise.