Quoting things from "WoW for Dummies" doesn't really count as contributing.
Yes, expertise capping sounds pretty nice. But not when taken into consideration that to reach a cap, any cap, you WILL have to sacrifice another stat. And that very stat could actually be of more value to your overall dps.
And I'm pretty sure that in most cases when it comes to expertise, being undercapped (assuming that this stat is being replaced with more pts in haste or crit) will be more beneficial for your overall dps, than overcapped by the same amount of pts.
It's not like passing the poison hit cap, where you will still see a small dps increase due to the additional white blows.
About the Assassination spreadsheet 1.3.4; Any plans of adding weapon enchant Black Magic? I'm pretty confident that at some point it surpasses both Mongoose, Accuracy and perhaps even Berserking.