My way of looking at it (which may be overly simplistic) is this. The further you round down, the more the spokes (tend to) become difficult to thread, which is a gradual process. Rounding up, however takes you closer to the situation where the spoke's thread runs out. This isn't a gradual process, it's a brick wall (which you won't encounter until later in the wheel-building process). Personally therefore, I'd rather err on the rounding-down side of things.
On my latest wheel-build (last week), I calculated the ideal spoke length at 264.9, and ordered 264's, (rounding-down by 0.9mm). I subsequently ended up using a rim with a 1mm larger ERD than originally planned. I had therefore effectively rounded-down the spoke length by a further 0.5mm (so 1.4mm total). Did this cause any lacing problems? Not at all! The wheel still went together like a dream, suggesting there is some safety margin there.