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 $61.38   6.153%   $65.1567   $65.16   $65.15   $65.20  
  Purchase #2 $87.49   3.344%   $90.4157   $90.42   $90.40   $90.40  
  Purchase #3 $227.49   13.841%   $258.9769   $258.98   $259.00   $259.00  
  Purchase #4 $60.94   9.611%   $66.7969   $66.80   $66.80   $66.80  
  Purchase #5 $134.71   8.420%   $146.0526   $146.05   $146.05   $146.10  
  Purchase #6 $143.34   8.602%   $155.6701   $155.67   $155.65   $155.70  
  Purchase #7 $135.32   3.129%   $139.5542   $139.55   $139.55   $139.60  
  7 purchase totals = $850.67       $922.6231   $922.63   $922.60   $922.80