Note: I know they might not strictly be interchangeable, but I might end up using "Furigana" and "Ruby text" in an interchangeable manner in this question.
I'm in the process of writing a short essay in Japanese. I'm going to be preparing two versions: one with Ruby text on all Kanji that are new to me (for further self-study purposes), and one with Ruby text on a subset of the more complex Kanji used.
When I craft a Japanese sentence and decide to place Furigana by a specific Kanji, do I need to use Hiragana each time?
I've noticed, in the dictionaries I use at the very least, that Onyomi readings are listed in Katakana and Kunyomi readings in Hiragana. Does this style carry across to essays and similar types of documents?
For instance, if I want t to use the kanji 上 in a sentence and explicitly mark the reading as うえ I would use Hiragana in my ruby text. But what if I wanted to use 日 in a sentence and explicitly mark the reading as ニチ? Would I still use Hiragana then?
I know that these are stupid examples, but I think that they still illustrate my question enough.