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 $9.99   3.724%   $10.362   $10.36   $10.35   $10.40  
  Purchase #2 $38.88   7.747%   $41.892   $41.89   $41.90   $41.90  
  Purchase #3 $31.39   6.594%   $33.4599   $33.46   $33.45   $33.50  
  Purchase #4 $249.35   13.260%   $282.4138   $282.41   $282.40   $282.40  
  Purchase #5 $15.77   13.530%   $17.9037   $17.90   $17.90   $17.90  
  Purchase #6 $159.55   5.395%   $168.1577   $168.16   $168.15   $168.20  
  Purchase #7 $26.30   4.357%   $27.4459   $27.45   $27.45   $27.40  
  7 purchase totals = $531.23       $581.6350   $581.63   $581.60   $581.70