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 $60.41   9.292%   $66.0233   $66.02   $66.00   $66.00  
  Purchase #2 $16.66   11.452%   $18.5679   $18.57   $18.55   $18.60  
  Purchase #3 $245.74   6.324%   $261.2806   $261.28   $261.30   $261.30  
  Purchase #4 $204.75   5.743%   $216.5088   $216.51   $216.50   $216.50  
  Purchase #5 $143.60   5.847%   $151.9963   $152.00   $152.00   $152.00  
  Purchase #6 $33.26   11.836%   $37.1967   $37.20   $37.20   $37.20  
  Purchase #7 $76.35   6.790%   $81.5342   $81.53   $81.55   $81.50  
  7 purchase totals = $780.77       $833.1078   $833.11   $833.10   $833.10