Unless you really need to free up a bit of processing power, or if it's some goofy required standard, there's no reason to convert to ProRes at all. You don't gain any dynamic range, resolution, or extra color by doing so; since h.264 inherently has way less color information than ProRes, you might as well work with the original footage. Otherwise, you're just adding unnecessary steps to your workflow.
If you HAVE to convert, denoise the footage first. That way you aren't tying up your processor denoising the ProRes footage in real time. But again; there's no qualitative reason to do so unless you just want to free up processing power during the edit.
The only advantage is if you want to keep anything you add to the cut in 10bit-- dissolves, titles, graphics, color correction so there is actually an advantage, as it will prevent further banding if there is any in the h.264.