From time to time, I fill in for our campus pastor on stage at the church. We have about 800 to 1000 people in attendance over the course of two services in a given weekend. On top of that, our church uses a campus model. That basically means that we have several campuses broadcasting the same message to about 10,000 people each week, so consistency in message is important.
Lots of organizations today have a similar issue. They have a large, diverse and disbursed base that they need to communicate a consistent message too. All things considered, that's pretty easy to do with the web these days.
The problem comes in with people. Sure, you can "enforce" every word that comes out of your peoples' mouths but it sounds that way. In sales, the individual style of the rep is often times the factor that wins the deal. Recently, when selecting some new technology to partner with, the style of the rep was what "broke the tie" between two very similar technology choices.
Back to my church for a second. While they place a high value on consistency in message, they place an equally high value on "sounding like yourself". When preparing remarks that I'll use on stage, I'm free to be myself within some guidelines. This makes the message delivery sounds more genuine, which leads people to listen and receive it more effectively. This, of course, leads to greater buy in and continued attendance.
The same principles apply to coaching your sales reps. Give them guidelines, but don't give them exactly what to say. Let your reps be themselves and do what they do best, deliver your message and build relationships with soon-to-be clients.