In G/L Setup you have Amount Rounding Precision and Unit-Amount Rounding Precision. You should look at those fields. Those fields aren’t displayed in G/L Setup you must edit it directly in table or make it visible. If you are using currencies you should look at Exchange Rates.
you set your decimal place formats to 0.00001 or however many you want, or also in the format 2:5 depending upon where you are looking, although all currencies are rounded to a set international standard as you do not have a half of your lowest denomination, hence the rounding.
You must always round. If I’m not in mistake in Navision maximum decimal fields in line amount is 5. I don’t know any country that doesn’t have a standard in terms of rounding. For example in Portugal line rounding and invoice rounding is defined in 2 decimal places.