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 $62.76   6.154%   $66.6223   $66.62   $66.60   $66.60  
  Purchase #2 $30.78   13.103%   $34.8131   $34.81   $34.80   $34.80  
  Purchase #3 $94.71   4.231%   $98.7172   $98.72   $98.70   $98.70  
  Purchase #4 $53.36   7.560%   $57.394   $57.39   $57.40   $57.40  
  Purchase #5 $182.27   9.318%   $199.2539   $199.25   $199.25   $199.30  
  Purchase #6 $174.61   4.248%   $182.0274   $182.03   $182.05   $182.00  
  Purchase #7 $40.74   13.517%   $46.2468   $46.25   $46.25   $46.20  
  7 purchase totals = $639.23       $685.0747   $685.07   $685.05   $685.00