Multishot is superior when there are 2 or 3 mobs. As soon as there are more Volley is better. Treat it as the same as you would in a normal situation.
The reason Volley is better is that the game can't determine which target gets the threat misdirected first, so it takes all of them. So for instance the little spiders in Naxx would all get sent to the tank with the first volley of Volley.

The following ticks are obviously going to you, but by then the tank has done various AoE things. Generally MD and Volley produces no problem for the Hunter, and I mainly do it for the benefit of the other raiders. If we can AoE sooner the sooner the pack is down the faster we can go on.