The installation instructions for my Sharp panels I had a few years ago said minimum 5 degrees for rain drain. Warned about puddles on the panel making for "hot spots" (ISTR that was the term) and damage to cells.
EDIT Read up some on that. Hot spots caused by shading with uneven heating. "Self-cleaning" by rainfall to clear off dust--most say at least 10 degrees. Panels have drain holes that should be kept clear.
My Canadian Solar brand panel installation guide does not mention tilting for rain like my old Sharp one did.
I have nearly flat panels on the Class C roof, and have seen how rain puddles make for dirty areas after they dry if the panels are dusty before it rains. That might lead to shading related hot spots if not washed. I clean my panels fairly often anyway.
