Footleg
In my experience only Martins method (border line completely independent of wall (or other) lines) results in a scrap that will be resilient to future edits. It is the only method I use now, after some bad experiences.
Firstly an independent border stands out as an entity to any future author (although perhaps is a little messy). It is not hidden in the drawing.
Secondly and most importantly, the creation of an area adds an id to any lines that make up its border. If in future a wall that also happens to be part of a border is split or redrawn, the id is removed (at least for one part of the line) and there is a very high chance that the area will no longer be drawn. I have alluded to this in http://therion.speleo.sk/wiki/doku.php?id=drawingchecklist#scrap_drawing
Bruce