You use Rend before CS because CS does not improve the damage of Rend so it's a waste of debuff time to apply Rend during it. The reason for the MS, Rend, CS, MS, OP, OP opener is that you get two OPs during the first CS (Also you stack LttS faster than opening with something that isn't MS).
For AoE, if you're specced into Blood and Thunder you want to get that up first obviously, at least if the mobs are going to live for a while. One thing that is good to know about blood and thunder is that if you use it to refresh Rend it will cause Rend to immediately tick on all targets in addition to the thunderclap damage.
Follow this up with Bladestorm (don't couple Bladestorm with Sweeping Strikes unless the mobs won't last longer than the duration of Bladestorm)
Now use Sweeping Strikes and resume the normal rotation but replace Slam with Thunderclap if there's enough targets to make it worthwhile. Remember to use Heroic Leap while Sweeping Strikes is up because it mirrors all the hits (Sweeping Strikes also mirror all the Thunderclap hits). Use Heroic Strike over Cleave while SS is up because Cleave only gets one extra hit and 2 HS hits does more damage than 4 Cleave hits.
I'm fairly sure you shouldn't WW as Arms, it just doesn't seem worth it. Depending on amount of targets this may actually not be true if you have Gurthalak since each WW hit can proc a tentacle.
It's also going to depend a bit on what things look like. For example, on Yor'sahj you may want to use Sweeping Strikes before Bladestorm while the mobs gather up and then use Bladestorm when they're all in place.