It is a good practice to save your work as you are working on the project. This is helpful to avoid a situation where you might lose precious data in the event of a system crash.
Saving the file in a PSD or Photoshop Document would preserve the layers of the file and its editing capabilities. Saving a separate copy of the file in a jpeg format would be useful for uploading it on the web.
If the image is based largely on text and would be printed on standard paper, saving it in PDF format is possible too.