Saving at 100% quality will never restore whatever loss has already occurred, and the image from your camera is usually at 95-97 quality. If you save at 100% quality you can as well use a lossless format such as PNG or TIFF (any compression except Jpeg).
In practice, quality depends on intended usage.
- If you want to keep the picture as a source for further edits(*), good quality is important, otherwise JPEG compression artifacts are going to get in the way (when doing selections, etc...). So keep the quality at 95 and make sure that the "Subsampling" (see the "advanced options" in the JPEG export dialog) is set to
4:4:4 (best quality).
- If it is a final version (for the web, or print), you can easily lower the quality to 80-85 and use a more aggressive subsampling: 4:2:2 or even 4:2:0. Tick the
Show preview in image window option to see in real time the influence of the JPG options on the final output (this also gives you an instant evaluation of the resulting file size) and adjust to your liking. "Sharper" pictures (or pictures with text added) will often require a better quality setting.
(*) while you work on a picture with Gimp, save it as XCF of course, to keep all the layers/channels/paths/selection.