It seems that it could be spelled either こんゆう or こにゅう, which might
depend on how it's pronounced. Is there a more correct
choice, or is it situational? Or does it not make any
difference, since it'd generally be written in kanji anyway?
How would you spell 今夕 in hiragana?

悪寄贈 神聞
November 7, 2015

Shirakawa-T
November 8, 2015
Hello Samuel.
It is spelled こん・ゆう not こにゅう, which has no meaning whatsoever, except that it is a name of a place. The difference between こん・ゆう and こにゅう is very significant.
I am sure you are aware of the on and kunyomi in a kanji. Except for tsu 「つ」 the pronunciation is not merged.
Take the word 一体 [ittai, one body; ...what on earth etc.]. The kanji "hito.tsu" has "itsu" as one of the onyomi. "Karada" has "tai" in it as it's onyomi. You don't say itsu-tai but ittai.
Kon.yaku 婚約 [engagement] is another example. The two kanji you see only has kon and yaku as the only onyomi (no kunyomi). It is spelled kon.yaku NOT koniyaku. The pronunciation is different.
It is spelled こん・ゆう not こにゅう, which has no meaning whatsoever, except that it is a name of a place. The difference between こん・ゆう and こにゅう is very significant.
I am sure you are aware of the on and kunyomi in a kanji. Except for tsu 「つ」 the pronunciation is not merged.
Take the word 一体 [ittai, one body; ...what on earth etc.]. The kanji "hito.tsu" has "itsu" as one of the onyomi. "Karada" has "tai" in it as it's onyomi. You don't say itsu-tai but ittai.
Kon.yaku 婚約 [engagement] is another example. The two kanji you see only has kon and yaku as the only onyomi (no kunyomi). It is spelled kon.yaku NOT koniyaku. The pronunciation is different.

悪寄贈 神聞
November 8, 2015
Okay, thanks. I'd seen it that way on Tangorin and I
generally wrote it that way here, but RJ seemed to usually ask for
a combination.