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 $56.84   13.580%   $64.5589   $64.56   $64.55   $64.60  
  Purchase #2 $22.57   11.533%   $25.173   $25.17   $25.15   $25.20  
  Purchase #3 $250.62   4.452%   $261.7776   $261.78   $261.80   $261.80  
  Purchase #4 $213.92   7.486%   $229.9341   $229.93   $229.95   $229.90  
  Purchase #5 $112.99   9.275%   $123.4698   $123.47   $123.45   $123.50  
  Purchase #6 $40.50   13.529%   $45.9792   $45.98   $46.00   $46.00  
  Purchase #7 $67.85   4.739%   $71.0654   $71.07   $71.05   $71.10  
  7 purchase totals = $765.29       $821.9580   $821.96   $821.95   $822.10