Using only /%postname%/ as your permalink structure is not recommended because it can cause a variety of errors. Changing your URLs will affect all posts and it will both break links and hurt your search engine rankings. Using a 301 redirect can help with this though.
Using only %postname%
If you use postname as the only element in your permalinks to create a structure such as example.com/post-title, the rewrite rules may make it impossible to access pages such as your stylesheet (which has a similar format) or the /wp-admin/ folder [is this true in WordPress 2.0+ versions?]. It’s best to include some numeric data (e.g. the post ID or date) in the permalink to prevent this from happening. Additionally, WordPress v1.2.x requires the use of a date structure in order for some features, such as the calendar, to function properly. /%year%/%monthnum%/%day%/%postname%/ is always a good start.
And I can tell you from experience: not funny! It was a few versions back, but I’m not going to try anymore. id/postname works just fine.
I use /%post_id%/%postname%
Did you read the structure tags? You can use something else, like month, date, or something. The advice is to use at least one value in the permalink other than the postname (category/postname works too).