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 $244.88   9.714%   $268.6676   $268.67   $268.65   $268.70  
  Purchase #2 $223.42   6.389%   $237.6943   $237.69   $237.70   $237.70  
  Purchase #3 $24.32   6.758%   $25.9635   $25.96   $25.95   $26.00  
  Purchase #4 $117.71   8.678%   $127.9249   $127.92   $127.90   $127.90  
  Purchase #5 $116.70   7.814%   $125.8189   $125.82   $125.80   $125.80  
  Purchase #6 $173.66   6.562%   $185.0556   $185.06   $185.05   $185.10  
  Purchase #7 $62.76   10.149%   $69.1295   $69.13   $69.15   $69.10  
  7 purchase totals = $963.45       $1,040.2543   $1,040.25   $1,040.20   $1,040.30