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 $27.75   4.123%   $28.8941   $28.89   $28.90   $28.90  
  Purchase #2 $74.89   8.489%   $81.2474   $81.25   $81.25   $81.20  
  Purchase #3 $30.41   3.559%   $31.4923   $31.49   $31.50   $31.50  
  Purchase #4 $32.63   3.313%   $33.711   $33.71   $33.70   $33.70  
  Purchase #5 $51.50   9.179%   $56.2272   $56.23   $56.25   $56.20  
  Purchase #6 $245.97   12.614%   $276.9967   $277.00   $277.00   $277.00  
  Purchase #7 $13.34   10.474%   $14.7372   $14.74   $14.75   $14.70  
  7 purchase totals = $476.49       $523.3059   $523.31   $523.35   $523.20