Really varies for me.
Factory wheels such as Rallye, Magnum 500 and road wheels, I typically prefer white letters, especially On Polyglas tires.
Torque thrust, black
Cragars raised white or black, really depends on the color of the car.
Keystone Klassics - Black, the white letter distracts from the wheels.

I do like white outlined letters on tires, but you don’t see them very often anymore. They will work with most aftermarket wheels and colors, more of an accent since they aren’t as bold as a full white letter.