Answer:
It is always better to stretch when you are warmed up, but it also depends how much is your plantar fascia inflamed or how painful it is. Over stretching can damage the plantar fascia.
I have plantar fasciitis myself for a few months now and I understood that treatment efficiency is very individual. If something works for one it will not necessarily work for the other. Today as I am feeling much better with the pain I am doing a combination of stretching and strengthening exercises.