Comment by zie

You need to keep state, so that when something goes wrong you can figure out that something went wrong.

PG's rounding can be summed up as: we round to the nearest digit you specify, but how we get there is mostly undefined.

Every bank and organization you do business with(especially if you cross-jurisdictions) will likely round pennies differently. PG and the decimal data type(which is actually the numeric data type) are only part of the solution, as they won't handle rounding pennies correctly for you, but they will store it, if you round properly yourself.

PG also has a money data type, but it also doesn't let you specify the rounding rules, so you have to be careful. Also, money is tied to lc_monetary, which is handled outside of PG's control, so you have to be very careful with it, when moving data across PG instances, or you might be surprised. Also, it doesn't let you specify which currency this money is, which means you have to store that yourself, if you ever have more than one currency to care about.

If you don't care about balancing to the penny with your external organizations(banks, CC processors, etc), you are 100% guaranteed to have a bad time eventually.