I always charged for render time but billed it as editing time, afterall, that's what it really is. Rendering is part of the process and since your editing system is being used to render, it's not generating cash flow from other projects.
I think it isn't a good strategy to use the slowest computer if you are charging the render to your client costs. The client wants a fast cheap work, if you give him what he wants he will return and you will earn more money. In time you will earn more money that if you take charge of a long rener time in one project.