Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $180.31   4.388%   $188.222   $188.22   $188.20   $188.20  
  Purchase #2 $181.33   7.860%   $195.5825   $195.58   $195.60   $195.60  
  Purchase #3 $164.90   3.756%   $171.0936   $171.09   $171.10   $171.10  
  Purchase #4 $225.89   8.202%   $244.4175   $244.42   $244.40   $244.40  
  Purchase #5 $227.75   5.354%   $239.9437   $239.94   $239.95   $239.90  
  Purchase #6 $155.46   7.779%   $167.5532   $167.55   $167.55   $167.60  
  Purchase #7 $169.34   13.691%   $192.5243   $192.52   $192.50   $192.50  
  7 purchase totals = $1,304.98       $1,399.3368   $1,399.32   $1,399.30   $1,399.30