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 $246.42   13.611%   $279.9602   $279.96   $279.95   $280.00  
  Purchase #2 $245.62   10.329%   $270.9901   $270.99   $271.00   $271.00  
  Purchase #3 $64.32   7.854%   $69.3717   $69.37   $69.35   $69.40  
  Purchase #4 $101.84   12.855%   $114.9315   $114.93   $114.95   $114.90  
  Purchase #5 $224.61   13.546%   $255.0357   $255.04   $255.05   $255.00  
  Purchase #6 $38.55   13.136%   $43.6139   $43.61   $43.60   $43.60  
  Purchase #7 $14.68   5.203%   $15.4438   $15.44   $15.45   $15.40  
  7 purchase totals = $936.04       $1,049.3469   $1,049.34   $1,049.35   $1,049.30