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 $17.77   12.531%   $19.9968   $20.00   $20.00   $20.00  
  Purchase #2 $53.53   6.869%   $57.207   $57.21   $57.20   $57.20  
  Purchase #3 $204.31   4.556%   $213.6184   $213.62   $213.60   $213.60  
  Purchase #4 $9.61   5.821%   $10.1694   $10.17   $10.15   $10.20  
  Purchase #5 $239.86   13.456%   $272.1356   $272.14   $272.15   $272.10  
  Purchase #6 $127.53   11.475%   $142.1641   $142.16   $142.15   $142.20  
  Purchase #7 $193.71   3.350%   $200.1993   $200.20   $200.20   $200.20  
  7 purchase totals = $846.32       $915.4906   $915.50   $915.45   $915.50