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 $42.99   10.258%   $47.3999   $47.40   $47.40   $47.40  
  Purchase #2 $141.75   7.764%   $152.7555   $152.76   $152.75   $152.80  
  Purchase #3 $239.43   4.407%   $249.9817   $249.98   $250.00   $250.00  
  Purchase #4 $190.67   3.441%   $197.231   $197.23   $197.25   $197.20  
  Purchase #5 $43.50   7.229%   $46.6446   $46.64   $46.65   $46.60  
  Purchase #6 $236.60   5.136%   $248.7518   $248.75   $248.75   $248.80  
  Purchase #7 $67.86   4.508%   $70.9191   $70.92   $70.90   $70.90  
  7 purchase totals = $962.80       $1,013.6836   $1,013.68   $1,013.70   $1,013.70