You aren't showing your hand by bidding before the last second.
At times you are. If you try to bid at the last second but the current bid is already over your maximum, then you will never bid. By not bidding at all, nobody knows you are interested in that item.
It's not really a big deal generally, but it might be if you are bidding on rare items. If you lose at $X, then other people might assume that's what you are willing to pay the next time, and bid just a little bit higher.