When i started using Memento & Link to Entry fields, i didn't use the parent → child link because the list of children in the parent was very substandard with no sorting, grouping, or filtering capability, so I used myself and recommended to others to link parent ← child & get the rich entries list that Memento provides while operating out of the child library.
But some years after, Vasiliy made the full-feature entries list available in the parent → child link as well, and many people prefer to operate primarily in the parent library.
My current recommendation is to not maintain both links -- too difficult to maintain. Instead, determine in what library you want to primarily operate. This is generally when the current, operational (transactional) library is your primary. In this case, link that library to the other.
Remove the other link field, so you won't have to main it redundantly.