I charge my full rate for everything including render time as I only use one computer and the CAD/Render program is not available for other jobs at the time. I should note though that most of my renders are fairly quick, none of those hour++ times one often hears about. I think if I had to have huge render times I would get a second computer for that so I could continue to work on my main computer. Then I would charge half my rate for rendering time on any render heavy work, since I would still need to cover the cost of the other computer, power, etc.