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.
The explanation in my dictionary is:
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 time, 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 have stopped being used in favour of forms derived from いく such as いって and いった.
They both mean the same of course, and there is plenty of info you can find explaining that ゆく is an older version. In addition, I have found that ゆく tends to be a more informal version of いく. Perhaps ゆく is also easier to pronounce than いく. I don't hold any data, but FWIW, I think ゆく is used more often than いく is conversation.