I think we use the dollars-and-cents system as the role model for lots of uses. That means two digits to the right of the decimal. Mostly, that is no great problem. I remember years ago, on the job, when a unit head had the computer programmers change his print-outs to carry three to the right instead of two. That was silly, because if the programmers accurately reflected our reality, instead of having two to the right, they would have none to the right. That means rounding off to the nearest whole unit. But each usage is different. There may be specialized uses in which six digits to the right is proper.