My experience in consumer banking says that every instrument specifies the precision of the calculation, how and when rounding happens, and slew of
little details.
So, yes, everyone has to understand how all their partners are doing rounding and summing.
In certain areas of the institutional finance world, everyone seems to accept that everyone's math is allowed to differ by a few cents, and they tally up the errors and move on with their lives.
I would, however, by quite surprised if my personal bank account did this.
So, yes, everyone has to understand how all their partners are doing rounding and summing.