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 $11.99   3.396%   $12.3972   $12.40   $12.40   $12.40  
  Purchase #2 $25.31   12.338%   $28.4327   $28.43   $28.45   $28.40  
  Purchase #3 $240.88   10.552%   $266.2977   $266.30   $266.30   $266.30  
  Purchase #4 $242.77   11.333%   $270.2831   $270.28   $270.30   $270.30  
  Purchase #5 $81.28   3.319%   $83.9777   $83.98   $84.00   $84.00  
  Purchase #6 $52.75   8.724%   $57.3519   $57.35   $57.35   $57.40  
  Purchase #7 $107.93   9.783%   $118.4888   $118.49   $118.50   $118.50  
  7 purchase totals = $762.91       $837.2291   $837.23   $837.30   $837.30