Sammy,

In my somewhat limited experience (five wheelsets so far) it's always better to round down than to round up. The danger in having the spoke even a small fraction of a millimeter too long is that the nipple runs out of spoke thread before achieving the proper tension. Rounding down to the nearest millimeter still gives you plenty of thread grip without any thread showing below the nipple, but ensures you won't bottom out before achieving the desired tension. My $.02.