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 $123.73   9.507%   $135.493   $135.49   $135.50   $135.50  
  Purchase #2 $242.63   8.736%   $263.8262   $263.83   $263.85   $263.80  
  Purchase #3 $135.96   4.379%   $141.9137   $141.91   $141.90   $141.90  
  Purchase #4 $30.51   11.666%   $34.0693   $34.07   $34.05   $34.10  
  Purchase #5 $83.28   10.128%   $91.7146   $91.71   $91.70   $91.70  
  Purchase #6 $144.76   8.728%   $157.3947   $157.39   $157.40   $157.40  
  Purchase #7 $78.44   12.372%   $88.1446   $88.14   $88.15   $88.10  
  7 purchase totals = $839.31       $912.5561   $912.54   $912.55   $912.50