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 $15.90   5.516%   $16.777   $16.78   $16.80   $16.80  
  Purchase #2 $37.72   11.480%   $42.0503   $42.05   $42.05   $42.10  
  Purchase #3 $99.62   3.810%   $103.4155   $103.42   $103.40   $103.40  
  Purchase #4 $64.46   9.158%   $70.3632   $70.36   $70.35   $70.40  
  Purchase #5 $132.25   9.336%   $144.5969   $144.60   $144.60   $144.60  
  Purchase #6 $78.48   3.462%   $81.197   $81.20   $81.20   $81.20  
  Purchase #7 $81.50   8.873%   $88.7315   $88.73   $88.75   $88.70  
  7 purchase totals = $509.93       $547.1314   $547.14   $547.15   $547.20