Hi Cristine,
mentioning the Check report got me curious… The control “LineAmount + LineDiscount” has the formatting properties:
AutoFormatType=1
AutoformatExpr=GenJnlLine.“Currency Code”
resulting in using either the G/L setup amount decimal places, or those set up in the currency code used on the gen. journal line. The amount decimal places can be changed to 2:2, this won’t affect the amount rounding precision. If you have access to the object designer you can change it at any time (I would recommend that you’re single user of the database, though). After restarting NAV the new decimals regime takes effect and you will not see more than 2 decimals on LCY amounts, even if they were posted with greater precision.
Changing the rounding precision (especially decreasing the precision) would practically invalidate the integrity of the G/L (and others) due to different rounding. Therefore NAV prohibits this when entries are already posted.
I tried this out on a real database: Changed the amount decimal places to 0:0, restarted, and voila, no decimals anywhere 
You can’t enter decimals either. Changed it back to 2:2, restarted, and everything was back to normal.
with best regards
Jens