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 $206.87   10.678%   $228.9596   $228.96   $228.95   $229.00  
  Purchase #2 $46.55   5.706%   $49.2061   $49.21   $49.20   $49.20  
  Purchase #3 $76.52   5.546%   $80.7638   $80.76   $80.75   $80.80  
  Purchase #4 $67.71   11.848%   $75.7323   $75.73   $75.75   $75.70  
  Purchase #5 $148.98   10.773%   $165.0296   $165.03   $165.05   $165.00  
  Purchase #6 $125.87   4.528%   $131.5694   $131.57   $131.55   $131.60  
  Purchase #7 $225.90   7.847%   $243.6264   $243.63   $243.65   $243.60  
  7 purchase totals = $898.40       $974.8872   $974.89   $974.90   $974.90