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 $7.90   3.446%   $8.1722   $8.17   $8.15   $8.20  
  Purchase #2 $42.70   5.556%   $45.0724   $45.07   $45.05   $45.10  
  Purchase #3 $11.73   6.308%   $12.4699   $12.47   $12.45   $12.50  
  Purchase #4 $28.33   4.586%   $29.6292   $29.63   $29.65   $29.60  
  Purchase #5 $191.87   12.663%   $216.1665   $216.17   $216.15   $216.20  
  Purchase #6 $231.99   3.338%   $239.7338   $239.73   $239.75   $239.70  
  Purchase #7 $32.74   8.250%   $35.4411   $35.44   $35.45   $35.40  
  7 purchase totals = $547.26       $586.6851   $586.68   $586.65   $586.70