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 $221.27   3.775%   $229.6229   $229.62   $229.60   $229.60  
  Purchase #2 $57.65   12.708%   $64.9762   $64.98   $65.00   $65.00  
  Purchase #3 $36.83   3.730%   $38.2038   $38.20   $38.20   $38.20  
  Purchase #4 $31.92   9.821%   $35.0549   $35.05   $35.05   $35.10  
  Purchase #5 $72.99   12.301%   $81.9685   $81.97   $81.95   $82.00  
  Purchase #6 $170.50   8.475%   $184.9499   $184.95   $184.95   $184.90  
  Purchase #7 $6.36   5.317%   $6.6982   $6.70   $6.70   $6.70  
  7 purchase totals = $597.52       $641.4744   $641.47   $641.45   $641.50