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 $15.75   7.870%   $16.9895   $16.99   $17.00   $17.00  
  Purchase #2 $11.97   4.682%   $12.5304   $12.53   $12.55   $12.50  
  Purchase #3 $143.80   12.337%   $161.5406   $161.54   $161.55   $161.50  
  Purchase #4 $34.71   6.523%   $36.9741   $36.97   $36.95   $37.00  
  Purchase #5 $14.77   11.664%   $16.4928   $16.49   $16.50   $16.50  
  Purchase #6 $18.59   13.587%   $21.1158   $21.12   $21.10   $21.10  
  Purchase #7 $171.46   9.511%   $187.7676   $187.77   $187.75   $187.80  
  7 purchase totals = $411.05       $453.4108   $453.41   $453.40   $453.40