Zephar:
I use 36 cells for my panels, I like to compensate for cloudy days or just to start my charging process earlier in the morning and later in the evening.
From my experience I only get 16.5V before the charge controller for 36 cells, this is because of heat and wire losses. My recommendation would be to be around 17.5V to 19.5V open circuit voltage.
Picmacmillan mentions that the series/parallel does not work for some people, I think it works fine if your panels have less cells in series, but If you wire enough cells in series from the beginning, your always meet your charging voltage.