Directory Submissions will certainly help you in the
SEO work of your website. Directories give Backlinks which are really important in order to get good rankings in Search Engines. However, it is suggested that you should do Regular submissions of your website into the Directories to get better rankings.
For example, it is better to Submit your website into 15-20 Directories Regularly rather than submitting your website into 500 Directories at a time in a Month. You will definitely get better rankings if you do the Directory submissions regularly.