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 $15.38   3.386%   $15.9008   $15.90   $15.90   $15.90  
  Purchase #2 $145.99   11.255%   $162.4212   $162.42   $162.40   $162.40  
  Purchase #3 $123.79   12.219%   $138.9159   $138.92   $138.90   $138.90  
  Purchase #4 $139.26   8.277%   $150.7866   $150.79   $150.80   $150.80  
  Purchase #5 $76.80   10.421%   $84.8033   $84.80   $84.80   $84.80  
  Purchase #6 $45.90   12.724%   $51.7403   $51.74   $51.75   $51.70  
  Purchase #7 $208.62   3.397%   $215.7068   $215.71   $215.70   $215.70  
  7 purchase totals = $755.74       $820.2749   $820.28   $820.25   $820.20