i would choose a new answer possibility ;D
'it depends on the map'
for example:
my map will have a system similar to TEve ORPG because it fits my map theme. it has an nearly open world (splitted in different cities/areas) but not completely and the dungeouns are somewhere inside (houses, caves, etc) => being teleported into the dungeon is the best way to do it.
if your map should be completely open so every unit can walk everywhere and should suddenly run into a boss/a 'dungeon' the way Ragos did it (even though i never played that map)....
so it depends on your map and the style it should have. ;D