I did not see any change to the sales conversion rate after adding BBB. However I had other trust (authority) graphics on the site.
D&B also told me the same thing - many companies requesting our company report (which was pot possible at that time). I have always questioned the validity of the data in D&B because a lot of it is self-reported (no audit or verification process). I also have experience purchasing the D&B credit builder service (which can be manipulated to help build one's credit score).
Listing of your site in D&B may be an extremely small factor in Google's algorithm but I don't think it is worth the expense. I would prefer to spend the money on the Yahoo Directory and Best of The Web directory.
On your site you need some type of major authority "seal" to gain the confidence of site visitors. D&B would be a very low priority one. TrustE, McAfee Secure are perhaps the top ones.
Last advice about D&B - make certain supplier invoices are paid perfectly on time and your credit score should increase over time. Your D&B credit score is critical in being able to obtain open account terms from suppliers.