As part of our LegalZoom LLC package, we opted for a free DUNs number, because it was free. I got a call from D&B today stating that it was ~$290 for us to have a credit file with them, and another ~$500 for the privilege of being able to look at that file, and add information to it for transactions that are not automatically reported. Our business model is contractual software development for small companies. So my questions are as follows:
Simple answer: No you don't.
DUNS is a private business credit rating agency just like Experian is for consumers. They are going to collect and report info no matter what you do. In fact one of their hard sell pitches is to hint that "the only way to ensure accuracy on your account is to have a subscription".
The only reasons you would need a DUNS number is because you want to do a credit check on someone else OR if customer contract requires it (very, very rare).
Net net: You should only do this if it benefits you.