Android: drawable resolutions

I've been through this post (and others) as well as through the documentation about supporting different screen resolutions in Android, but I couldn't find a clear answer to a (simple) question:

Is it ok to just use "res/drawable" for images in an android app?

Background: The only images that are needed in this specific app are the app icon itself and an icon for a notification, there won't be any images in any layout.
So in my understanding, if no "hdpi"-, "mdpi"- and "ldpi"-folders are found, Android will use "res/drawable" as the fallback.
As the only pitfall with different screen-resolution seems to be that Android will scale images for a specific resolution if no special one is found, this should only be a problem when UPscaling, because the image will get blurry. But if I provide all "hdpi"-images in "res/drawable" (instead of 3 different ones), won't Android just DOWNscale those images if the size is too big?
If that's true, I could save some APK-space by just a third of the images.

Follow-Up question: I read that for API-level 3 a dir by the name "drawable-v3" is required. Is that true or is "drawable" the fallback for this API-level also?

Any hint is appreciated.