It helps if you're familiar with your spoke calculator.

From experience you'd know where the spokes tend to come to in the nipple. Knowing that you can determine how much tolerance you have on either side of the target length and then decide accordingly on which direction to err.

For example if your spokes always tend to come very close to the top of the nipple, you'd probably prefer to round down, or if your prior builds tend towards the short side, then you'd round up.