A good question.
The easy answer:
If the only changes you want to make to your site are ones that you can make via the theme's settings (of which there are many), then just stick with using the theme's built-in settings and call it good.
However, if you wish to make changes to your theme that aren't available through the theme's settings, then you'll need to use a child theme.
In either case, you should never alter the theme files themselves.
The not-so-easy answer:
It depends. You can make a lot of changes via the theme's settings, but those changes get added as additional css to each page that loads. If you've only made a few changes, that doesn't add much to your site's load times. (The same thing happens with the custom css function.) If you start making extensive changes through the theme settings, or adding a lot of custom css, it can slow down your site. In that case, you would want to consider using a child theme, because most of your customisations will go into a single style sheet which will be loaded once per site visit, rather than once per page load. It's a way of speeding up your site a bit.
The nice thing is that you don't have to choose with Graphene. If you find that you are making a lot of changes via the theme's options, you can always use the theme's export function to export those changes into a custom made child theme stylesheet. (Yes, Graphene is really that cool.) So you can have the best of both worlds.