Fabrizio;

If you pre-wet film, you don't have to worry about water droplets, but if you do not, a few drops of water can leave spots on the film due to uneven development. And yes, the pre-wet can cause a very very tiny change in the film density, but it is uniform and cannot be seen except by measurement with a densitometer. With drops of water, you can visually see the unevenness.

PE