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 $7.89   12.651%   $8.8882   $8.89   $8.90   $8.90  
  Purchase #2 $114.52   12.684%   $129.0457   $129.05   $129.05   $129.00  
  Purchase #3 $117.72   7.779%   $126.8774   $126.88   $126.90   $126.90  
  Purchase #4 $157.31   4.725%   $164.7429   $164.74   $164.75   $164.70  
  Purchase #5 $116.28   5.610%   $122.8033   $122.80   $122.80   $122.80  
  Purchase #6 $48.43   3.611%   $50.1788   $50.18   $50.20   $50.20  
  Purchase #7 $79.28   9.239%   $86.6047   $86.60   $86.60   $86.60  
  7 purchase totals = $641.43       $689.1410   $689.14   $689.20   $689.10