I don't think there is a general guideline for this. It also depends on the purpose of the map, I think.
I'm in the same position as you: I have a large-scale 'overall' map, and I am going to map out specific parts in ever increasing detail as I go along. I intend to use the same style for all the maps, to create a unified 'feel' of the (series of) maps.

However, if the characters get handed a map by an NPC, I think I'd use one of the hand-drawn styles instead of the default (detailed) style.