sometimes areas are adjacent, sometime one area is inside another one, other time areas intersect each other.
When tracing the new area adjacent to an existing area, for the new border points, is better (or not) to reuse points from the existant adjacent area?
Reusing points avoid space in between the two adjacent areas, but complicate further editing.
Is there a directive about this?
You should try and map things as accurately as possible, to match what is “on the ground”. If areas are next to each other, with no space in between, then map them as such. So you can map them as overlapping ways, sharing nodes between areas.
It is true this may make editing a bit more complicated, but most editors have features to help. eg you can use the “Follow” feature in JOSM or Potlatch, to draw a way sharing nodes with another way. Also in JOSM, you can middle click on the way, to select each way in turn.
Note that it is sometimes be better to map adjacent areas with a multipolygon relation http://wiki.openstreetmap.org/wiki/Relation:multipolygon
eg you have one way for each shared part of the boundary, then add it a relation for each area. But relations can also complicate editing, so maybe not helpful to use them for every little overlapping area. Though if you have an area with a ‘hole’ in it, then it is necessary to use a multipolygon.