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 $185.83   11.268%   $206.7693   $206.77   $206.75   $206.80  
  Purchase #2 $62.49   3.417%   $64.6253   $64.63   $64.65   $64.60  
  Purchase #3 $94.38   8.380%   $102.289   $102.29   $102.30   $102.30  
  Purchase #4 $108.32   13.517%   $122.9616   $122.96   $122.95   $123.00  
  Purchase #5 $226.27   4.858%   $237.2622   $237.26   $237.25   $237.30  
  Purchase #6 $50.99   11.643%   $56.9268   $56.93   $56.95   $56.90  
  Purchase #7 $180.35   4.757%   $188.9292   $188.93   $188.95   $188.90  
  7 purchase totals = $908.63       $979.7634   $979.77   $979.80   $979.80