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 $51.84   8.421%   $56.2054   $56.21   $56.20   $56.20  
  Purchase #2 $83.62   10.524%   $92.4202   $92.42   $92.40   $92.40  
  Purchase #3 $239.43   9.380%   $261.8885   $261.89   $261.90   $261.90  
  Purchase #4 $106.69   4.721%   $111.7268   $111.73   $111.75   $111.70  
  Purchase #5 $100.68   8.168%   $108.9035   $108.90   $108.90   $108.90  
  Purchase #6 $202.63   10.192%   $223.282   $223.28   $223.30   $223.30  
  Purchase #7 $149.72   11.437%   $166.8435   $166.84   $166.85   $166.80  
  7 purchase totals = $934.61       $1,021.2699   $1,021.27   $1,021.30   $1,021.20