I'm not really certain how individual targeting works. I'm not even certain individual targeting works at all. They might just target the center of the enemy unit and random scatter is determined by the computer.

However, deploying in deep blocks does have one drawback: the unit will only fire when everyone is in range, so they'll have to get closer to the enemy before they fire. This can result in you taking unanswered casualties. Personally, I tend to deploy my missile troops 4-6 ranks deep in most cases. They aren't so wide they become unwieldy and are still shallow enough not to make much of an impact on unit range.