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 $61.47   7.292%   $65.9524   $65.95   $65.95   $66.00  
  Purchase #2 $70.69   11.293%   $78.673   $78.67   $78.65   $78.70  
  Purchase #3 $60.33   11.379%   $67.195   $67.20   $67.20   $67.20  
  Purchase #4 $176.59   5.702%   $186.6592   $186.66   $186.65   $186.70  
  Purchase #5 $171.78   9.177%   $187.5443   $187.54   $187.55   $187.50  
  Purchase #6 $59.42   12.129%   $66.6271   $66.63   $66.65   $66.60  
  Purchase #7 $153.71   6.491%   $163.6873   $163.69   $163.70   $163.70  
  7 purchase totals = $753.99       $816.3383   $816.34   $816.35   $816.40