Control the rounding level of currencies
Promotion engine rounds the discount value to the known decimal digits of the order currency. The official decimal digits of currencies are defined by ISO 4217 standard, however as a business, you might have different requirements. For example, normally, you will only see $12.34 , but it's not very uncommon for you to have a price of $12.3456. This might cause some problem where something that should be free, is not, or even worse, the actual cost of something (like shipping cost) will be negative, causing validation error.
That is, however can be worked around by controlling the number of decimal digits for a currency. In the example above, it might make senses to set CurrencyDecimalDigits of USD to 4. This can be done by this snippet code added to an initialization module, which has side-wide affect:
var f = Currency.USD.Format; f.CurrencyDecimalDigits = 4; Currency.SetFormat(Currency.USD.CurrencyCode, f);
This is, however, not without drawback. Any amount in USD will have 4 decimal digits, so they might not be displayed nicely to end user. I would suspect most end users would like to see USD 12.34, instead of USD 12.3456 in their cart page. That is something you might to think about.
We have some changes that will fix the rounding issues completely (I hope!), but that requires breaking changes so it will have to wait until Commerce 13.
For now, the workaround above should work.