In this answer, the person who answered used $\tau$ instead of $2\pi$, and I commented that he should probably use $2\pi$ instead to avoid confusion.
My question is:
Do we have any guidelines here on math.SE about $2\pi$ vs $\tau$?
There's no formal guideline about using $\tau$ instead of $2\pi$. But anyone who uses $\tau$ should remember that it is not standard notation. In particular, the notation $\tau$ for $2\pi$ is not used in textbooks, and those with little mathematical knowledge are unlikely to know what $\tau$ is supposed to mean.
At the same time, if someone does use $\tau$ in an answer, I don't think anyone else should edit the answer just to remove it. A comment simply stating that $\tau$ is $2\pi$ should be enough.