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 $226.71   10.284%   $250.0249   $250.02   $250.00   $250.00  
  Purchase #2 $249.71   11.228%   $277.7474   $277.75   $277.75   $277.70  
  Purchase #3 $45.86   10.481%   $50.6666   $50.67   $50.65   $50.70  
  Purchase #4 $175.67   5.810%   $185.8764   $185.88   $185.90   $185.90  
  Purchase #5 $146.83   10.149%   $161.7318   $161.73   $161.75   $161.70  
  Purchase #6 $241.60   10.365%   $266.6418   $266.64   $266.65   $266.60  
  Purchase #7 $77.51   10.811%   $85.8896   $85.89   $85.90   $85.90  
  7 purchase totals = $1,163.89       $1,278.5785   $1,278.58   $1,278.60   $1,278.50