If a system wraps data, it will give what are called vertical rounding errors where the sum of the converted figures does not equal the converted total.
Some packages offer account level conversion.
I have seen one case where doing that allowed the entry of unbalanced transactions, putting the trial balance out. The price list will suffer from the price accordion effect in Ireland before base conversion to euro, everywhere else after that. The Association of British Insurers came to the same conclusion. Secondly, by continuing to calculate and store monetary data as pounds sterling, our systems will introduce aggregative and multiplicative errors, when converting from one currency to the other in the course of normal daily processing.