It would be sane that:

1) Preferences > Reset to Builtin Default Values
2) File > Open Recent > Clear Menu

performs a complete clearing of the Scene Builder preferences.

Today operation #2 only clears the list of recent files.
It keeps the preferences associated to the recent files.
So "Recent Files" list may be empty but some default values
remains in the preferences db. I think this discrepancy
might confuse the user.