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 $19.31   3.547%   $19.9949   $19.99   $20.00   $20.00  
  Purchase #2 $78.94   3.556%   $81.7471   $81.75   $81.75   $81.70  
  Purchase #3 $178.69   5.489%   $188.4983   $188.50   $188.50   $188.50  
  Purchase #4 $52.55   11.585%   $58.6379   $58.64   $58.65   $58.60  
  Purchase #5 $241.32   12.391%   $271.222   $271.22   $271.20   $271.20  
  Purchase #6 $158.63   12.124%   $177.8623   $177.86   $177.85   $177.90  
  Purchase #7 $153.49   8.441%   $166.4461   $166.45   $166.45   $166.40  
  7 purchase totals = $882.93       $964.4086   $964.41   $964.40   $964.30