DNG files do not contain some metadata supplied by the camera, which can be used by the manufacturer's software. Thus, if you don't keep the original Raw photo, you will lose this information.
If your 1000% sure you're never going to use the manufacturer's software, then this isn't a problem. But who can be sure what software you will be using 10 years from now?
If you ever expect to enter any photo contests that later require you to also submit the RAW unedited file, you will need it and not your converted/edited DNG file.