object.reserve() and array size

[Lars T. Kyllingstad]

Is object.reserve() only useful for large arrays, or should I always use 
it if I intend to append to an array?

Let's say I want to read some data, and I expect there to be ~100 bytes 
of data.  Is there any point in using reserve() first, or will there 
always be that much memory available to an array?

  byte[] buffer;
  buffer.reserve(100);
  foreach(byte b; dataSource) buffer ~= b;

-Lars