White letters on painted steel wheels on 1970 and later like the factory did it looks best for me.
Black walls, or redlines, on 1969 and earlier cars.
Mag wheels it can go either way depending on the rim design, type of body style, color of the car ect.
If the car has newer billet/machined type wheels I tend to go with blackwalls.
If we are talking Keystones, Cragar SS ect. then white letters seem to belong with those older wheel designs better.