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 $239.26   12.251%   $268.5717   $268.57   $268.55   $268.60  
  Purchase #2 $83.73   7.600%   $90.0935   $90.09   $90.10   $90.10  
  Purchase #3 $236.38   4.716%   $247.5277   $247.53   $247.55   $247.50  
  Purchase #4 $248.74   3.383%   $257.1549   $257.15   $257.15   $257.20  
  Purchase #5 $43.67   6.300%   $46.4212   $46.42   $46.40   $46.40  
  Purchase #6 $10.57   10.129%   $11.6406   $11.64   $11.65   $11.60  
  Purchase #7 $63.83   13.727%   $72.5919   $72.59   $72.60   $72.60  
  7 purchase totals = $926.18       $994.0015   $993.99   $994.00   $994.00