What is the preferred symbol to use for population proportion?

Question

I've seen p, P and $\pi$ used. My gut feeling is that $\pi$ is the best to use, to be consistent with other Greek letters used for population parameters ($\mu, \sigma, \rho, \beta_0, \beta_1,$, etc.) but I am looking for some guidance - like from the ASA or someplace with some clout. Has a consensus been reached in academia?