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 $19.55   4.786%   $20.4857   $20.49   $20.50   $20.50  
  Purchase #2 $9.44   7.262%   $10.1255   $10.13   $10.15   $10.10  
  Purchase #3 $231.46   5.400%   $243.9588   $243.96   $243.95   $244.00  
  Purchase #4 $20.50   6.544%   $21.8415   $21.84   $21.85   $21.80  
  Purchase #5 $102.41   10.393%   $113.0535   $113.05   $113.05   $113.10  
  Purchase #6 $143.40   7.826%   $154.6225   $154.62   $154.60   $154.60  
  Purchase #7 $241.73   9.438%   $264.5445   $264.54   $264.55   $264.50  
  7 purchase totals = $768.49       $828.6320   $828.63   $828.65   $828.60