'default' folder and untrusted nodes

Is it possible to avoid sharing ‘default’ folder unless explicitly added on both sides? I noticed there is no ‘Ignore’ button, only ‘Later’ in sharing prompt that appears when I add new device.

Or should I just sync default folder with everybody and don’t store anything important there?

The default folder, like any other folder, is not shared unless explicitly added on both sides. You get the prompt on one side because you explicitly added it on the other.