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 $16.63   9.513%   $18.212   $18.21   $18.20   $18.20  
  Purchase #2 $81.64   9.520%   $89.4121   $89.41   $89.40   $89.40  
  Purchase #3 $81.76   4.267%   $85.2487   $85.25   $85.25   $85.20  
  Purchase #4 $114.45   5.238%   $120.4449   $120.44   $120.45   $120.40  
  Purchase #5 $228.73   10.591%   $252.9548   $252.95   $252.95   $253.00  
  Purchase #6 $78.96   3.510%   $81.7315   $81.73   $81.75   $81.70  
  Purchase #7 $58.33   6.181%   $61.9354   $61.94   $61.95   $61.90  
  7 purchase totals = $660.50       $709.9394   $709.93   $709.95   $709.80