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I see no point to do so on files that are digitally released (as on author's pixiv, twitter or whatever). If JPEG artifacts are moderate, just post the original and use jpeg_artifacts tag; if users want a "cleaner" version they can download and process it themselves.

If the artifacts are very heavy, chances are waifu2x is also going to kill some details in order to remove these excessive artifacts - at least that's my experience.

Generally speaking, it's normally better to keep more "pristine" versions of these images, since no processing is perfect or lossless.

I do found it's useful in processing scans, particularly when the screenings are small in scale (either low DPI scans, or high quality printing).