That's a good question and one I've been asking myself lately. I used to tweak most of my animation in the graph editor, which is the way I learned it at school. Keeping the curves clean and organized was the way to go, and a workflow I'm still following. All of your animation is represented through curves, and even though you have multiple options of how to edit your animation, your curve editor will give you the most detailed information about your animation.
And yes, I would keep curves clean. It makes it a lot easier to edit and reorganize sections of your animation. It's like your work area. If it's messy, you will have a hard time finding specific items. You know roughly where it is, but if the area is clean and organized, you will have a much easier time. Same goes for curves.
It will also be a lot easier for other animators to deal with your shots if you keep curves clean. There are moments when another animator has to work on your shot, either because you are too busy with other shots or because you are no longer on that show, and if you are one of those animators working on someone else's shot, you will appreciate it when the curves are clean. Part of being a professional animator is not just being able to animate well, but to be conscious of the show pipeline and company structure. You have to be aware that other people will deal with your files, so keep them organized, including your curves.
However, I've noticed recently that I rarely go into the graph editor. The only time I manipulate curves is for moving holds and technical fixes. The rest of the time I work out my animation through my viewports and set keys to edit my animation. I build overlap and other details into my poses during blocking, so I have less and less need to edit the animation in the graph editor. By keeping my curves clean and by organizing my keys, I can work out the timing by shifting around keys in the Maya timeliner, which also lets me convert my curves from linear to spline.
In a few years, I will probably have another way of animating, but this is my current approach to curves.