I think you are right. I also worked on a bill-tracking software and one of the first things we noticed is that each company has their own way of rounding total

I think you are right. I also worked on a bill-tracking software and one of the first things we noticed is that each company has their own way of rounding totals. There is no one correct way. For example one company rounds each line to three digits internally for the total but prints only two digits on each line. For my software I started using decimaljs for the highest precision but in the end it never made any difference cent wise even when the payments go in the millions.