Why would it be? It's a sound being made by the tongue and throat, so it most closely relates to speech, but doesn't get the quotes since it's not actual formed words. Of course, there's another rule that says you italicize words when they are referring to the word, rather than the definition of the word. So maybe since tsk is an onomatopoeia, referring to the sound is basically referring to the word as well? I do think I remember a few books that have italicized tsk. Personally, started off thinking I wouldn't italicize it, but now I'm not sure.
I read a lot of books as a kid in which italics were used when someone was yelling. I grew up with the impression that italics = loud. So, unless the tsk sound is really loud, I don't think italics are appropriate.
As a general rule, the only times I use italics in my writing are 1) something in a foreign language and 2) book titles. If I'm fortunate to have my ms published some day, it may well be that the publisher will choose to italicize some other things. But for now, that is how I am writing them.
