I have always washed the model, before assembly and again before the painting. (for pre-paint, I use Plastic Prep, until my last bottle runs out)
I used to use warm water for the wash, and warm water for the rinse. But, just about a year ago, I changed that to warm soapy water for the washing, and cold water for the rinsing. I did that based on a blog post I read about how soap is best absorbed in water. A warm rinse mixes with the soapy water you are trying to rinse off, and a cold rinse chases the soapy water off without mixing in with it, which is the whole reason for rinsing in the first place.
Rex