I'm mostly curious about 5e but tagging this General because "always round down" is such a longstanding tradition in D&D.
Actually, it's not that "long standing", it was just part of the general improvements made by 3e to harmonise behaviour across the system, as before that there were almost as many ways of rounding as there were rules.
What's the point? Why not "always round the way your 3rd-grade math teacher taught you?"
I don't think I've ever seen the reason actually written down, but I think it's because of the bonus to the ability scores. They wanted only even ability scores to give you a bonus (because it's much simpler to center the curve around the number 10 than 11), and that meant rounding down. Since (as explained above), they wanted only one method for the whole game system, it's the one that was chosen.
As far as I can tell, the main effect that "always round down" has on the game is that dealing half damage benefits the defender... to the tune of 1 point of damage, 50% of the time. Well, that, plus an annoying amount of "(round up)" exceptions throughout the text.
Are there really that many ? I honestly cannot recall even one, but I'm sure you'll let us know.
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