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 $238.55   5.125%   $250.7757   $250.78   $250.80   $250.80  
  Purchase #2 $128.72   11.151%   $143.0736   $143.07   $143.05   $143.10  
  Purchase #3 $192.54   12.605%   $216.8097   $216.81   $216.80   $216.80  
  Purchase #4 $138.38   4.650%   $144.8147   $144.81   $144.80   $144.80  
  Purchase #5 $70.49   11.827%   $78.8269   $78.83   $78.85   $78.80  
  Purchase #6 $15.91   11.324%   $17.7116   $17.71   $17.70   $17.70  
  Purchase #7 $116.41   7.382%   $125.0034   $125.00   $125.00   $125.00  
  7 purchase totals = $901.00       $977.0156   $977.01   $977.00   $977.00