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 $139.31   3.395%   $144.0396   $144.04   $144.05   $144.00  
  Purchase #2 $20.38   3.617%   $21.1171   $21.12   $21.10   $21.10  
  Purchase #3 $138.46   7.663%   $149.0702   $149.07   $149.05   $149.10  
  Purchase #4 $27.63   3.132%   $28.4954   $28.50   $28.50   $28.50  
  Purchase #5 $136.96   3.703%   $142.0316   $142.03   $142.05   $142.00  
  Purchase #6 $11.43   13.770%   $13.0039   $13.00   $13.00   $13.00  
  Purchase #7 $132.64   6.288%   $140.9804   $140.98   $141.00   $141.00  
  7 purchase totals = $606.81       $638.7382   $638.74   $638.75   $638.70