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   5.640%   $20.3991   $20.40   $20.40   $20.40  
  Purchase #2 $137.85   3.230%   $142.3026   $142.30   $142.30   $142.30  
  Purchase #3 $197.53   13.744%   $224.6785   $224.68   $224.70   $224.70  
  Purchase #4 $223.85   13.354%   $253.7429   $253.74   $253.75   $253.70  
  Purchase #5 $104.47   3.389%   $108.0105   $108.01   $108.00   $108.00  
  Purchase #6 $209.73   3.299%   $216.649   $216.65   $216.65   $216.60  
  Purchase #7 $19.80   10.526%   $21.8841   $21.88   $21.90   $21.90  
  7 purchase totals = $912.54       $987.6667   $987.66   $987.70   $987.60