In a few places, with some ubercool neighbors, we have several harvestable squares with no armies on them. All the neighbors gather with care not to exhaust herbs and minimize bumps.
I don't mind anyone gathering from my sovereign squares either.
Where I do not have close neighbors, I keep an army on resources I want. This is to -
1. Avoid wandering critters occupying the square.
2. Avoid someone else sending an army from a far away city to the square and telling me the square now squarely belongs to him.
2 is very likely to happen if you have valuable resources near. I sympathize with what Mogul and Binky said. But unless you have some agreements with neighbors, keep an army on the resources you want. If you take back an occupied square from an alliance bigger than yours, they will call you a bully and come after you.