JPEG uses lossy compression. If you are going to send an image via e-mail, it should be as small as possible. JPEG files are also viewable in web browsers. Avoid opening and saving a JPEG file multiple times since a JPEG file loses some quality each time. It is a bit like making a video tape copy from a copy.
Every generation is of lower quality.
If you want a master copy, keep it in TIFF and you can always save a JPEG file later for email or for placing on a web page.