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 $19.80   10.777%   $21.9338   $21.93   $21.95   $21.90  
  Purchase #2 $196.63   3.340%   $203.1974   $203.20   $203.20   $203.20  
  Purchase #3 $34.64   11.219%   $38.5263   $38.53   $38.55   $38.50  
  Purchase #4 $197.82   9.261%   $216.1401   $216.14   $216.15   $216.10  
  Purchase #5 $15.89   12.634%   $17.8975   $17.90   $17.90   $17.90  
  Purchase #6 $75.56   9.166%   $82.4858   $82.49   $82.50   $82.50  
  Purchase #7 $6.33   4.571%   $6.6193   $6.62   $6.60   $6.60  
  7 purchase totals = $546.67       $586.8002   $586.81   $586.85   $586.70