When you install WordPress, the default permalink structure you have is http://example.com/?p=123 . You can retain this structure if having the shortest URL possible is your priority. Else, you need to change this for better SEO.
The most used custom WordPress permalink structure is http://example.com/postname/ .
The permalink structure I use is http://example.com/year/month/postname/ . This is good for performance. Also, gives an idea to the user when the post was written. They can also trim the URL like http://example.com/year/month/ or http://example.com/year/ and see archives for that period.
If you write posts daily or write many posts a day like in news websites, then http://example.com/year/month/day/postname is a good structure.
If you want your posts to be timeless, do not include date information and just include the unique post id.
If you like your posts to be aggregated by Google News, use Google News SEO module for the WordPress SEO plugin. You don’t need to have a special permalink format for this. Earlier, you needed to have a unique number in the URL for this.
Some people are over obsessed with having category name in URL for SEO. But I have seen this affecting blog structure / incompatibility with some plugins. Better to avoid using category names in URL.
If you are considering to change the permalink structure but worried about losing traffic, Yoast’s permalink change tool should help you.