When writing furigana I was told if it's written「フリガナ」write it in katakana. If it's「ふりがな」then write it in hiragana.
I'm curious if there's a standard or everyone that makes a document just kind of chooses if they want it in hiragana or katakana?
Yes, when the form says
ふりがな you usually have to fill it with hiragana, and when the form says
フリガナ you usually have to fill it with katakana. Both styles are common. In online forms there may be an explicit instruction or an example.
If you are a form designer, you can basically choose whichever you like, but there are several considerations:
I feel forms/programs developed recently mainly use furigana in hiragana.
I found a website advising you how to fill in your full name when you are asked for the furigana.
It's safe to assume that rule would be followed elsewhere.
Other forums I checked had posters who said the same thing. However, there do exist people who are not sure which to use, or have a preference for one kana. カタカナ does seem more official and important-looking from what I could gather. On the other hand, the habit of using ひらがな as ふりがな is very common. I can post more links to these discussions if you like.
If you need further convincing, you can read this psychology blog where the author takes for granted that same system of using the same script as the one 'furigana' is written in - in effect, answering 'yes' to your question. For such an assumption to be made, it's only natural to conclude the rule is wide-spread.
The blog writer suggests it is common for the person filling in the form to copy the model given by the other person (who wrote the form). What follows are humorous examples that test that pyschological impulse where doing so might be considered absurd and contrived.
I don't know of any examples where one would write the furigana differently than the kana reading. Doing otherwise seems confusing to me.
Kanji represent meaning while kana represent sound. This is why there are often multiple possible kanji for a given sound, and a given kanji might be read in multiple ways. Furigana is a reading aid so the reader knows which reading was intended.
Remember, in general, one is not required to write furigana. It is only written as an aid to the reader. Thus if you do not know if a word would be written in hiragana or katakana, or if you do not know which reading was intended, it might be a good idea to omit the furigana to avoid an error.
One extra note, from what I know, Japanese children learn hiragana before katakana. Thus all furigana written for children of that age are in hiragana.
Both of the standard notation of furigana 「フリガナ(Katakana)」, 「ふりがな(Hiragana)」, in general Japanse documents especially for pronounce of name, address.
In my childhood, hiragana notation is used for Kanji, katakana notation is used for foreign words.
Some Japanese apps check characters according to furigana notation「フリガナ(Katakana)」,「ふりがな(Hiragana)」,「ｶﾀｶﾅ(Half-width Katakana)」.
In very old Japanese computing, there is only used Half-width kana and forced to use 「ｶﾀｶﾅ(Half-width Katakana)」.
My experience with ふりがな in general is that there are a few accepted standards. The most widely accepted standard is to use ひらがな for all kanji, unless the kanji is standing in for a word that is a loan word (for example, 功夫 -> カンフー). This kind of usage is not very common, but some words as this exist.
Another "standard" is mostly seen in academic settings, typically when teaching foreign students. To teach the difference between 音読み and 訓読み for kanji, you can use カタカナ for 音読み and ひらがな for 訓読み. Since 音読み tend to be borrowed readings derived from Chinese, it helps to see the difference in origin as you're learning vocabulary, and so this is often used as a tool in the classroom.