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 $136.98   12.836%   $154.5628   $154.56   $154.55   $154.60  
  Purchase #2 $21.32   8.588%   $23.151   $23.15   $23.15   $23.20  
  Purchase #3 $16.94   4.554%   $17.7114   $17.71   $17.70   $17.70  
  Purchase #4 $239.89   7.459%   $257.7834   $257.78   $257.80   $257.80  
  Purchase #5 $200.73   9.694%   $220.1888   $220.19   $220.20   $220.20  
  Purchase #6 $65.29   13.525%   $74.1205   $74.12   $74.10   $74.10  
  Purchase #7 $208.32   13.730%   $236.9223   $236.92   $236.90   $236.90  
  7 purchase totals = $889.47       $984.4402   $984.43   $984.40   $984.50