There are a couple of places where it would be useful to have reputation checks before proceeding - two that I've come across are Sporeggar in Zangarmarsh, and The Mag'har in Nagrand (this is probably a special case, because I faction changed after finishing Hellfire so had no reputation).

I've had a quick look at the way to implement it (and have some local -untested- changes in an old version) - there are two ways of handling it:

1. On a status change i.e going from neutral to friendly. This wouldn't work for the existing Zangarmarsh guide as it actually says to keep killing until you get 250 rep short of friendly (as you're about to turn in a quest to get you that last bit).
2. listen to the combat log events and parse any relating to reputation gains. This is a potentially high load task (and would definitely need to only be turned on if there was an active reputation step)

The other difficulty is that the current reputation values are only retrieved by basically passing the reputations dialog and matching up the name of the faction to its reputation - there isn't any direct api call. This index can be cached and used.

Adding this for a future todo rather than a high priority task.

This should now all be handled with the new REP tag.