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 $19.78   8.636%   $21.4882   $21.49   $21.50   $21.50  
  Purchase #2 $74.61   3.766%   $77.4198   $77.42   $77.40   $77.40  
  Purchase #3 $115.40   4.382%   $120.4568   $120.46   $120.45   $120.50  
  Purchase #4 $75.45   6.535%   $80.3807   $80.38   $80.40   $80.40  
  Purchase #5 $216.83   4.344%   $226.2491   $226.25   $226.25   $226.20  
  Purchase #6 $241.64   11.150%   $268.5829   $268.58   $268.60   $268.60  
  Purchase #7 $98.44   13.438%   $111.6684   $111.67   $111.65   $111.70  
  7 purchase totals = $842.15       $906.2459   $906.25   $906.25   $906.30