Ben Hutchings wrote:
If you're dealing with other people's money then there are regulations that specify the required intermediate precision and rounding rules, which may vary from place to place.
For market risk, the amounts are very high and somewhat 'virtual', because what is evaluated is a probability of gain/loss, and therefore some imprecision is acceptable. When it is about 'real' money, imprecision is unavoidable (...change rates...), but what helps is the small number of operations (couple of additions / multiplications) in each separate transaction. When other types of financial operations implies many numbers, regulations may enforce to have specific accounts storing rounding errors (differences between the actual and expected results): Although there are rounding errors, nothing 'disappeared' from the bank's books.