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 $225.67   12.420%   $253.6982   $253.70   $253.70   $253.70  
  Purchase #2 $187.95   5.689%   $198.6425   $198.64   $198.65   $198.60  
  Purchase #3 $11.99   7.253%   $12.8596   $12.86   $12.85   $12.90  
  Purchase #4 $146.36   7.531%   $157.3824   $157.38   $157.40   $157.40  
  Purchase #5 $10.46   4.729%   $10.9547   $10.95   $10.95   $11.00  
  Purchase #6 $129.56   7.505%   $139.2835   $139.28   $139.30   $139.30  
  Purchase #7 $20.90   12.724%   $23.5593   $23.56   $23.55   $23.60  
  7 purchase totals = $732.89       $796.3802   $796.37   $796.40   $796.50