pronunciation - When is 行く pronounced as いく, and when is it ゆく?

Are there any rules or guidelines as to when to pronounce 行く as いく or ゆく?

I looked it up on jisho.org, and the two pronunciations have the exact same definition. I tend to hear ゆく more often in songs, but that is just anecdotal.

Answer

The explanation in デジタル大辞泉 is:

「いく」の語形も上代からみられ、平安時代以降は「ゆく」と併用される。「ゆく」「いく」はほとんど意味は同じであるが、古くは「ゆく」のほうがより広く使われ、特に訓点資料・和歌（「生く」との掛け詞の場合を除き）では、ほとんどすべてが「ゆく」である。現在では「ゆく」に比べて「いく」のほうが話し言葉的な感じをもち、したがって、「過ぎ行く」「散り行く」など、文章語的な語の場合には「ゆく」となるのが普通である。なお、「ゆきて」のイ音便形「ゆいて」も用いられたが、現在は一般的でなく、促音便形は「ゆく」のほうは用いられず、「いく」を用いて「いって」「いった」となる。

My translation / synopsis is as follows:

いく has been seen from ancient times but from the Heian period both have been in use. いく has almost exactly the same meaning as ゆく but in olden times, ゆく was used more widely: Putting the use of double entendres and word play based on 生く aside, then the use of 行く in Japanese poetry (waka/tanka) or symbolism can almost always read as ゆく. As a result, when expressions such as 「過ぎ行く」「散り行く」 are used stylistically in written language they are normally read as ゆく. However forms such as ゆきて have stopped being used in favour of forms derived from いく such as いって and いった.