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 $20.61   8.474%   $22.3565   $22.36   $22.35   $22.40  
  Purchase #2 $197.79   13.186%   $223.8706   $223.87   $223.85   $223.90  
  Purchase #3 $182.26   9.458%   $199.4982   $199.50   $199.50   $199.50  
  Purchase #4 $201.97   3.425%   $208.8875   $208.89   $208.90   $208.90  
  Purchase #5 $174.83   3.302%   $180.6029   $180.60   $180.60   $180.60  
  Purchase #6 $15.28   12.271%   $17.155   $17.16   $17.15   $17.20  
  Purchase #7 $20.25   5.680%   $21.4002   $21.40   $21.40   $21.40  
  7 purchase totals = $812.99       $873.7709   $873.78   $873.75   $873.90