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 $250.63   3.796%   $260.1439   $260.14   $260.15   $260.10  
  Purchase #2 $236.38   9.557%   $258.9708   $258.97   $258.95   $259.00  
  Purchase #3 $18.93   7.669%   $20.3817   $20.38   $20.40   $20.40  
  Purchase #4 $77.80   10.218%   $85.7496   $85.75   $85.75   $85.70  
  Purchase #5 $167.95   4.747%   $175.9226   $175.92   $175.90   $175.90  
  Purchase #6 $6.61   6.137%   $7.0157   $7.02   $7.00   $7.00  
  Purchase #7 $54.50   7.565%   $58.6229   $58.62   $58.60   $58.60  
  7 purchase totals = $812.80       $866.8072   $866.80   $866.75   $866.70