I too use Photoflo. I remember reading somewhere that if the bubbles/rinse don't run down the negs, it's time to mix a fresh batch. I always check this and if the rinse sits on the negs, it gets put back into water and I mix a new batch, before rinsing again.
So far, I haven't had any problems, and it's so cheap, mixing new rinse every so often isn't a hardship.

Nanette