Who says 'always round down?' If I recall correctly that's an oversimplification of what Roger Musson actually says in the book. The rule is
Always use the rule-of-thumb for spoke number rounding - "up or down a maximum of +/- 1mm from the calculated length to the available spoke length.
It would be foolish to round down 285.5 to 284mm because you will end up with a set of spokes that are 1.5mm shorter than ideal which is sufficient to leave the neck of the nipple unsupported (see classic no-go diagram). This is a relatively common way for nipples to fail through fatigue, where the head cracks off the nipple. Far better to have .5mm of spoke protruding from the nipple than to be too short.
Similarly if you but 282s for where you require 283.2, the spokes are 1.2mm too short. To what end?
If you follow the rule of thumb you will buy 28x 284 and 28x 286mm and if all the measurements you are relying on are correct everything will be lovely.