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 $20.78   3.528%   $21.5131   $21.51   $21.50   $21.50  
  Purchase #2 $97.87   12.849%   $110.4453   $110.45   $110.45   $110.40  
  Purchase #3 $178.81   3.665%   $185.3634   $185.36   $185.35   $185.40  
  Purchase #4 $38.88   10.599%   $43.0009   $43.00   $43.00   $43.00  
  Purchase #5 $178.64   12.794%   $201.4952   $201.50   $201.50   $201.50  
  Purchase #6 $60.42   12.655%   $68.0662   $68.07   $68.05   $68.10  
  Purchase #7 $246.67   10.353%   $272.2077   $272.21   $272.20   $272.20  
  7 purchase totals = $822.07       $902.0918   $902.10   $902.05   $902.10