Listening to some of the older songs in which Koyasu sings, I realise that he really had a bad problem with intonation. Which is a little late, but in my defense his singing is good enough to make you forget that. I mean, that! voice! Which he uses to such! effect! But I digress. So I was poking about my song collection today, and listening to songs.
The earliest song he sang that I've heard is "Hikari Aru Sekai E", from Shurato. ("Paint it Black" is from the same period, so it doesn't count, I guess.) The song is sung by the main seiyuu (plural) of Shurato, and Koyasu, playing one of the biggest roles in that show, would definitely have to have a part. I would hazard, though, that the producers had some trouble with his singing, because although he does sing in it, he's "sheparded" by a more experienced singer, who sings with him in an effort to help keep him in tune. (It happens in band, so mayhap this device can be applied to voice?) And he manages, although there are still parts, particularly towards the end, where he isn't.
What I find weird is that the other singer has a voice several octaves above his, of a timbre that doesn't blend well, either. Sure, the singer was noticeably more experienced and skilled, but.. but. Weird. In "Paint it Black" he sings solo, and is rather out of tune, but it's not as bad as it could have been, given the song doesn't have much in the way of chordal support for his voice. His pronounciation of "black", however..
In his later songs, like those he did for Initial D, generally don't have any intonation flaws that I noticed, but it should probably be noted that I listened to "Black Out" several times over before realising that in parts he was singing in English. I think I've come across an Ini D fanpage which called it "black cow", once. Also, the song he did for Z/eta, "Defend the Truth", was it? I heard "defend the cheese~", and have never gone near the song since. (It was a nice song otherwise.)