Since rBf5e0dfe59c7e748, we have a simple & practical way of setting up defaults for DNA data. These defaults are used for the "Reset to Default Value" operator, available in the context menu for buttons.
Here's a helper script to extract the defaults: https://stackoverflow.com/a/55368203/432509
There are still a number of properties that don't have a default assigned. These can be reported here.
Note that these missing defaults typically appear in "groups" because the entire data-structure misses the default value definition. In that case there's no need to list each individual value, just mention where they are (e.g. Armature object, relation settings).
