Personally I think it has to do with vehicle look, stance and size of the sidewall on the tire. Those low profile tires, yeah sure blackwall out, if the tire is alittle taller and on a sportier type ride, maybe white letters out. WHITEWALLS??? never anymore, had a few the parents had whitewalls out when growing up and it really just didn't look right.
Big bubbly off road tires like swampers, yeah again, black walls out. I personally think it's a total package look on deciding on one or the other.
I don't think the letters out on some stockish trucks look too bad though.