Sometimes is useful to add a zero-offset skirt to prevent warping / unsticking of a part.
Typically that's when a part has long parallel tracks going in the same direction (e.g. the length of a rectangular bar).
And so the edges that REALLY require a skirt tied to them are those that, with more downwards force associated, will stop the part lifting off as those parallel tracks cool and contract.
However it may also be that some areas of the bottom of the part contain fine detail, detail which will be buried in excess filament if skirts are applied to all the edges concerned.
For support material it's possible to go into manual mode and remove / place bricks representing where support material will be generated.
Suggestion: provide a similar feature for zero-offset skirts in which you can select / deselect edges of the .stl model that are in contact with the surface of the bed.
Here's a simple example (attached). The part (viewed from underneath) has channels cut in the bottom: definitely don't want the skirt in there! At the same time the part is sort of long and thin and desperately needs an attached skirt to keep it from warping, particular as it contains a tall tower structure (partially visible).