Maximum quality is achieved when you edit the source files. However,
1) The difference in quality should be visually undetectable.
2) If the source files are H.264 or other highly compressed codec, it might take longer to render the edited sequence from H.264 compared to rendering from ProRes files.
3) If color correction is performed with a different application you should export using a low compression intermediate codec (like ProRes) for grading, or use a Dynamic Link between Premiere Pro and another Adobe app.