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 $6.36   4.342%   $6.6362   $6.64   $6.65   $6.60  
  Purchase #2 $202.92   7.429%   $217.9949   $217.99   $218.00   $218.00  
  Purchase #3 $158.90   6.506%   $169.238   $169.24   $169.25   $169.20  
  Purchase #4 $41.46   3.255%   $42.8095   $42.81   $42.80   $42.80  
  Purchase #5 $23.96   6.442%   $25.5035   $25.50   $25.50   $25.50  
  Purchase #6 $117.53   10.491%   $129.8601   $129.86   $129.85   $129.90  
  Purchase #7 $163.33   13.525%   $185.4204   $185.42   $185.40   $185.40  
  7 purchase totals = $714.46       $777.4626   $777.46   $777.45   $777.40