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 $5.71   11.770%   $6.3821   $6.38   $6.40   $6.40  
  Purchase #2 $177.74   7.116%   $190.388   $190.39   $190.40   $190.40  
  Purchase #3 $192.71   13.243%   $218.2306   $218.23   $218.25   $218.20  
  Purchase #4 $78.52   3.290%   $81.1033   $81.10   $81.10   $81.10  
  Purchase #5 $102.35   5.219%   $107.6916   $107.69   $107.70   $107.70  
  Purchase #6 $12.99   7.398%   $13.951   $13.95   $13.95   $14.00  
  Purchase #7 $30.83   8.507%   $33.4527   $33.45   $33.45   $33.50  
  7 purchase totals = $600.85       $651.1993   $651.19   $651.25   $651.30