I've been reading more novels in Japanese recently (かもめ食堂: recommended!), and have noticed that it's unusual for 言う to be written in kanji. Not just in set phrases like という and とはいえ but in attributions such as 「みどりはいった」。
My question: are there any situations where it's required or would be more natural or unambiguous to write 言う rather than いう? If I'm emailing a friend would it seem pretentious to use the kanji?