Like you, I roll with 'story appropriate' and don't really get into high detail about it. I tend to keep them more toward 'wild' places... if one ran through a city it would either be very faded, or very strong with the spirit of man. If one ran through downtown Seattle in my game, it would intersect with the Aztechnology arcology... and they don't need any more magic than they already have! (Hm, that would be a good plot point though... especially if the story was that they somehow 'bent' it to intersect with them...)
There was one out in the Barrens that had an ancient crow shaman squatting in an old building that was sitting along it in my last game. It was an interesting, random, side-story for the mage in my campaign

Crunch~