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 $234.81   6.478%   $250.021   $250.02   $250.00   $250.00  
  Purchase #2 $62.61   8.615%   $68.0039   $68.00   $68.00   $68.00  
  Purchase #3 $239.75   9.249%   $261.9245   $261.92   $261.90   $261.90  
  Purchase #4 $107.84   12.329%   $121.1356   $121.14   $121.15   $121.10  
  Purchase #5 $192.49   6.324%   $204.6631   $204.66   $204.65   $204.70  
  Purchase #6 $72.85   6.103%   $77.296   $77.30   $77.30   $77.30  
  Purchase #7 $42.46   5.586%   $44.8318   $44.83   $44.85   $44.80  
  7 purchase totals = $952.81       $1,027.8759   $1,027.87   $1,027.85   $1,027.80