Asset Postprocessor - Differentiate New & Old Assets?

Conceptually, I like the Asset Postprocessor feature in Unity. There are several cases where I'd like to post-process assets as our artists are adding assets to the project:

• Stop the mesh importer from automatically creating a materials folder.

• Force meshes to default to have "None" for the rig type.

• Have particular texture settings by default (compression off, mipmaps disabled, etc).

However, when tinkering with post-processors in the past, I noticed that these changes would be applied even after the asset is initially imported into the project. For example, I have a few cases where I would like texture compression turned on, but the asset post processor would eventually get its hands on that "special case" texture and revert its settings.

Using asset post-processors, is there a way to detect whether the asset being processed is new to the project or previously indexed/accounted for? Ideally, I could use the PostProcessor scripts to apply "sane" default settings for imported assets, but then modify their settings from there on a case-by-case basis.