Bah.  It doesn't matter who's consuming the rounding of a binary float to decimal microseconds:  there are only 32 possible fractional parts where nearest/even and half-up deliver different results.  half-up preserves properties of these specific inputs that nearest/even destroys.  These inputs themselves have no bias - they're utterly uniformly spaced.

Not only does nearest/even _introduce_ bias on these inputs by destroying these properties, it doesn't even preserve the spacing between them.  Half-up leaves them all 5 microseconds apart, while nearest/even creates a bizarre "sometimes 4 microseconds apart, sometimes 6" output spacing out of thin air.

So it's not a question of "when in doubt" to me, it's a question of "live up to what the docs already say".  Although, again, it doesn't make a lick of real difference.  That's why we'll never stop arguing about it ;-)