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 $17.44   3.613%   $18.0701   $18.07   $18.05   $18.10  
  Purchase #2 $17.32   3.225%   $17.8786   $17.88   $17.90   $17.90  
  Purchase #3 $139.27   13.354%   $157.8681   $157.87   $157.85   $157.90  
  Purchase #4 $114.29   10.325%   $126.0904   $126.09   $126.10   $126.10  
  Purchase #5 $171.67   9.463%   $187.9151   $187.92   $187.90   $187.90  
  Purchase #6 $220.74   12.869%   $249.147   $249.15   $249.15   $249.10  
  Purchase #7 $69.98   11.815%   $78.2481   $78.25   $78.25   $78.20  
  7 purchase totals = $750.71       $835.2174   $835.23   $835.20   $835.20