Here's how I see it.

Instead of a strict posting order, you should just encourage players to wait for everyone else their character is interacting with before posting themselves. If you're in a group with three other characters, then wait for those three before moving on, as opposed to just jumping in again after only just one.

The problem with a thread-wide posting order, though, is that when not all your characters are in the same place, it creates needless stalling. Take the example I brought up above, except there are four more characters in the roleplay, in another area. If you impose a strict posting order, then RPers in group A are waiting for posts from those in group B despite the fact that they shouldn't have to depend on group B's posts at all. In the worst case scenario, let's say that the posting order just so happened to line up so that everyone in group B had to post before group A could continue. That means group A has to completely halt its activity while it waits for everyone in group B, and vice versa, stopping them from running at a good speed by letting them both move at once.

Not to mention, with a strict posting order, everyone is completely held up if one person leaves, causing the entire RP to halt while the GM figures out a solution - as opposed to the alternative of just letting [i]a few[/i] people get held up, while the rest of the RP moves. The few who did get stuck can get caught up and meet up with the other players later without too much effort - it's better than letting the whole RP freeze and forcing people to wait for the solution to the drop-out of a character that they had nothing to do with, in my opinion.

In short, strict posting orders are best avoided. In fact, in most RP's, I place a rule telling RPers [i]not[/i] to wait for players they're not doing anything with, because it just slows things down and holds people up. It's much better to just encourage players to use common sense and wait for the people they actually need to wait for, and no one else.