I can't think of a reason not to test letters. It costs almost nothing and can yield huge results.
Writing is a Solitary Pursuit
The actual writing isn't 100 percent of the game. Preparation, attitude and editing may be more important.
You get briefed, see what worked and didn't work in the past, and what the competition is doing. You understand the product and the benefits of using it, you know what motivates the target audience, get an appreciation for the offer, and then you go off by yourself and sit down to write.
But before you do, you should review a list of things to avoid—things that can kill your letter before you even start. Here are a few:
Write to motivate your target audience. Writing to yourself is a common and major sin. Your prospects don't particularly care about your company; they care about what you can do for them. The letter should be written to them—the target audience. This is tough to do, especially when your bosses think they're your target audience, which of course, they aren't. If you have a serious difference of opinion, suggest one of those low-cost letter tests.
Avoid "corporate-ese" copy. It's stilted, boring and self-serving. Corporate-ese reads like a bad annual report. If you find yourself writing things that sound like mission statements (e.g., "in our ongoing efforts to maximize"), you're writing corporate-ese.
You don't have to write dry B-to-B letters. There's no such person as "business." There are only people at businesses, and to reach those people and motivate them, B-toB letters must trigger a personal reaction. A while back, 3M was selling presentation equipment through a seminar program about presentation skills, and it wasn't doing well. It asked us to fix it. Its original mailer had extolled the business benefits of things such as getting people to work well together, achieving consensus, having more productive meetings, etc. All were benefits to the recipient's company, about which the recipients really don't care.