I agree with Andy,
If I may add to this subject, I think that even in the case of incoming hostile reinforcements from the edge of the map, the scenario designer should edit the map and include a warning text to that (aprox) location.
cheers,
Pyros
p.s of course there are exceptions. As for example when there is an enemy base near the edge of the map, then you may consider the edges of the map near that base as a possible entry points for hostile patrols.
