While using a logic level shifter (of any kind - see bellow) is advisable, well manufactured NeoPixels can usually cope with the 3.3V data as the LOW/HIGH threshold sits somewhere round about 3V.
But lesser quality LEDs tend to have higher thresholds an hence won't reliably recognise 3.3V as HIGH.
As long the LEDs' power rails are connected to Vin and GND (or external 5V supply with common ground) you won't fry the GPIO on the Photon as the current draw of the NeoPixel data pin is neglible and no voltage is coming from there either. But you might see erronous colours on your LEDs when they are not happy with the level.
But you'll also not need a fancy level shifter chip to do the job. A simple transistor controlled via the GPIO switching 5V to the data pin will do.