Block is never rounded. 5 inefficient block is 2.5. If the enemy attack was 3, this would not be enough.
Page 8, “Block Phase”, 5b:
b. Inefficient Blocks are reduced by half. When totaling the final Block value, total the values of all inefficient Blocks, divide by two (round down) and then add the full values of all efficient Blocks.
Effectively it doesn’t make any difference. You just need to make sure to total all inefficient blocks before dividing.