It was actually something I removed sometime ago to make the module stable. I can't remember where the post on it is, on the forum, but it goes something like this.
In thirty bees we use 6 digit decimal precision for prices. So if you have an item that is $10 and you have a 33% discount on it, the real price is $6.666666, which would come out to $6.67. But there are so many different rounding methods people use, should they be rounded per line or per item. Like if someone purchased 2 of those products should the price be rounded off the single products then added together? Or should it be added together and then rounded? Then you get deeper in the hole when you start applying currency conversions and taxes too.
Basically the problem comes down to when you send products on an order over to paypal, paypal uses their own rounding system on those prices. If they come up with a 1 cent difference they will error out the transaction. Then that stops the customer from buying. Its a difficult system to work against, so the best fix was taking out the details. Then there is no chance for errors.