Plaintext is likely to be OK, however; that is, if you're using a "dumb and simple" true text editor and not one that tries to do fancy things like parse the text and perform syntax highlighting.
Has anyone audited "less" on Unix recently? These days it does a whole bunch of magic stuff under the hood to show special formats "correctly."
I have been known to use Python's IDLE with suspect text documents, not because it's bullet proof just because the user base has to be super low. (as a text reader)
f.open('suspect.txt', 'r')
print f.read()
Would the UNIX utility `cat` fall into that category?
cat is probably safe. I would be concerned about the terminal you are running it in though.