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 $21.55   10.809%   $23.8793   $23.88   $23.90   $23.90  
  Purchase #2 $100.50   6.542%   $107.0747   $107.07   $107.05   $107.10  
  Purchase #3 $74.47   9.292%   $81.3898   $81.39   $81.40   $81.40  
  Purchase #4 $104.86   3.427%   $108.4536   $108.45   $108.45   $108.50  
  Purchase #5 $66.87   7.538%   $71.9107   $71.91   $71.90   $71.90  
  Purchase #6 $148.59   9.145%   $162.1786   $162.18   $162.20   $162.20  
  Purchase #7 $149.68   9.615%   $164.0717   $164.07   $164.05   $164.10  
  7 purchase totals = $666.52       $718.9584   $718.95   $718.95   $719.10