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 $28.99   3.837%   $30.1023   $30.10   $30.10   $30.10  
  Purchase #2 $158.46   10.583%   $175.2298   $175.23   $175.25   $175.20  
  Purchase #3 $218.59   8.677%   $237.5571   $237.56   $237.55   $237.60  
  Purchase #4 $92.40   9.833%   $101.4857   $101.49   $101.50   $101.50  
  Purchase #5 $8.69   5.866%   $9.1998   $9.20   $9.20   $9.20  
  Purchase #6 $19.62   12.671%   $22.1061   $22.11   $22.10   $22.10  
  Purchase #7 $32.86   5.553%   $34.6847   $34.68   $34.70   $34.70  
  7 purchase totals = $559.61       $610.3655   $610.37   $610.40   $610.40