I agree with Kaga, a strict posting order can slow RPs down. In my opinion, it can sometimes be the death of a RP.

I was once a part of roleplay that handed players numbers. Number one would post first followed by two then three then so on and so forth, but somewhere down the road one of the players vanished and we had to pause the RP for a few weeks while the GM looked for a replacement. It was definitely a motivation killer, but that wasn't the only problem. We could never post when we felt like we wanted to or when we were free to do so because we had to wait for our turn. It was a problematic because sometimes we'd be free when it wasn't our turn but busy when it finally was, 

A looser posting order tends to do just fine, and it makes the RP move at a quicker pace. 

I prefer it when GMs create posting cycles.

Example: Post once every 2-4 days.

The GM's post is the start of a new cycle, the players post after the GM and if all of them gets a post in before 4 days the GM posts again, if not then the GM waits for the four day mark and posts again whether everyone else has posted or not. That way people need to be active or left behind, but it isn't chaotic and gives everyone time as well. If the GM feels like things have been too fast they can opt to wait a few days before starting another cycle. It's kind of like an order, but people are free to post anytime between the given amount of time.