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 $183.29   12.130%   $205.5231   $205.52   $205.50   $205.50  
  Purchase #2 $127.46   8.606%   $138.4292   $138.43   $138.45   $138.40  
  Purchase #3 $43.81   12.751%   $49.3962   $49.40   $49.40   $49.40  
  Purchase #4 $67.54   4.649%   $70.6799   $70.68   $70.70   $70.70  
  Purchase #5 $94.76   4.188%   $98.7285   $98.73   $98.75   $98.70  
  Purchase #6 $136.92   12.602%   $154.1747   $154.17   $154.15   $154.20  
  Purchase #7 $244.39   8.470%   $265.0898   $265.09   $265.10   $265.10  
  7 purchase totals = $898.17       $982.0214   $982.02   $982.05   $982.00