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 $19.99   9.719%   $21.9328   $21.93   $21.95   $21.90  
  Purchase #2 $164.56   7.479%   $176.8674   $176.87   $176.85   $176.90  
  Purchase #3 $110.80   3.758%   $114.9639   $114.96   $114.95   $115.00  
  Purchase #4 $35.76   13.692%   $40.6563   $40.66   $40.65   $40.70  
  Purchase #5 $13.48   13.242%   $15.265   $15.27   $15.25   $15.30  
  Purchase #6 $76.79   13.210%   $86.934   $86.93   $86.95   $86.90  
  Purchase #7 $43.62   5.264%   $45.9162   $45.92   $45.90   $45.90  
  7 purchase totals = $465.00       $502.5356   $502.54   $502.50   $502.60