The rationale behind recommending ею is that ей is easily confused with the dative. Nothing else has this problem, and it's already gone when using the form with a preposition. Dative only takes two prepositions, к and по, and both are exclusive to it. So the ней forms following prepositions are never ambiguous (yes, it's also the prepositional case form, but no prepositions overlap with instrumental there either), and there's no particular reason to use нею.
-ю endings instrumental sound archaic/poetic, and with literally everything except ею, they're not something a person would use nowadays unless they're deliberately trying to come across as either affected or facetious.
(I might just add that none of this applies to the 3rd-declension noun instrumental
-ью, where the
ю never goes away. That said, it is itself a contraction of the older
-ию — мыслию, совестию — which is even more distinctly archaic than the
-ею on 1st declension nouns, adjectives, and pronouns.)