When I was working at Bloomberg, one of the problems I faced is that JavaScript doesn't have a decimal numeric datatype. This is a drawback which even Douglas Crawford has pointed out.
Every time you want to display a price on the terminal, you really should spend time thinking about whether that price would be displayed correctly.
The particular rounding rules even vary from currency to currency. Some are even enforced by law, so bugs could leave you exposed to regulatory risks. And people tend to get touchy when you are dorking with their money.
What happened in practice was that everybody just ignored it--but sooner or later, some obscure but urgent bug would pop up. Inevitably, it was hard to trace and hard to fix, because there's no support there for the arithmetic which people actually use for money.
