I don't know what program you use to process JPGs, but in Lightroom, and several other programs, processing RAW is *EXACTLY* the same as procesing RAW. The only difference is there are a few more sliders and options that are not grayed out. If you couldn't see those sliders in Lightroom, you wouldn't know whether you had a JPG or a RAW loaded.
Custom WB is very helpful. In many instances. But in others it's nearly a waste of time. Many older artificial stadium lights cycle. And as they do, they change from a magenta to a green hue and back again. The WB of them is changing CONSTANTLY, and not all of the lights cycle at the same time. The indoor facility I shoot in most often does this, and I have to white balance every single photo independently in lightroom. I don't even bother with custom WB in that facility any more. I just set it on AUTO, shoot RAW, and adjust in post.