It has been proposed that a frequency extra terrestrials might choose to transmit a signal on, seeking other intelligent life, is "hydrogen times pi". More precisely, the hydrogen line frequency times pi (around 4.46 GHz). This would be a frequency that doesn't fall onto any other naturally occurring frequencies that might mask it out, will indicate intelligence and would be a frequency other intelligent life would think to look for.
Pi was chosen because it is irrational. Sqrt(2), phi, e and yes, tau, would have also worked. I personally think e is a fine choice, because there is absolutely no debate over whether a multiple or fraction of e should have been chosen instead. In terms of being "special" and "unique", in my mind it is right up there with pi and tau.
But here's the question you should ask yourself:
If someone from SETI came to you today stating that an intelligent message had been intercepted (and you believe him/her) and that this was on either pi or tau times the hydrogen line frequency and you had to guess which, like your life depended on it, what would you guess? Pi or tau?
Tau has my vote.