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 $18.54   13.350%   $21.0151   $21.02   $21.00   $21.00  
  Purchase #2 $196.93   5.625%   $208.0073   $208.01   $208.00   $208.00  
  Purchase #3 $20.84   9.223%   $22.7621   $22.76   $22.75   $22.80  
  Purchase #4 $55.66   13.522%   $63.1863   $63.19   $63.20   $63.20  
  Purchase #5 $74.85   4.171%   $77.972   $77.97   $77.95   $78.00  
  Purchase #6 $27.78   3.168%   $28.6601   $28.66   $28.65   $28.70  
  Purchase #7 $65.84   3.152%   $67.9153   $67.92   $67.90   $67.90  
  7 purchase totals = $460.44       $489.5182   $489.53   $489.45   $489.60