I guess some preload is OK, as long as heat expansion of balls/races still allows everything to spin "freely" when heated up.
But how would you know that except checking freeplay while it's still hot?
And disc hubs do get really hot sometimes (pad friction), more so then drums or the rear axle bearings.
That's why they say backoff a notch or such.
I'd rather have a bearing with "some" freeplay while hot, then one that's "binding".
Excessive preload, even on a quality bearing, will produce scoring, mainly on the race surfaces.