Hi Tones,
Thanks for the hint. I didn't know the clone button and it surely may help sometimes but it seems that custom buttons will better fit my needs.
I was successful in customizing the new-here button. I cloned it, named the new version as custom-new-here, and inserted tags={{!!tags}} in its actions. It works perfectly. When I create a new tiddler using the custom-new-here button it is created with its "parent tiddler" tags.
To replicate this behavior on new-journal-here I had to define a new value named inheritedTags
<$set name="inheritedTags" value={{!!tags}}>
and modify the definition of journalButtonTags() to get it
\define journalButtonTags()
$(journalTags)$ $(inheritedTags)$
\end
I removed the reference to [[$(currentTiddlerTag)$]] since I'm not interested in the name as a tag.