Just to clarify, are you asking when you should center elements or how to center them via flexbox?
If it’s a matter of when to center, I think really it depends on the layout and/or the content on a given page. I’ve dabbled a bit with flexbox, usually if you’ve got several elements I want displayed in a row, having them centered to me looks the best.
If you’ve got something like say a paragraph by itself, centering again works (though I’d stay away from large widths on the paragraph itself). If it’s say a paragraph and an image, having the text left or right aligned with the image is a classic.