"when I read an article which said that it's the best way to gain google rank"
That article lied to you, dear... again, I've been in this business for years - your URL means very little anymore, ESPECIALLY to google. I've seen sites with "p=4" rank better than those with a postname or "page.html"
What counts in ALL the search engines the most, is your content, and how popular your site is (traffic, relevant links to your site, etc.)
There's reason for it - and why you don't see many sites using keywords and hyphens in their domains anymore - because that was a "trick" used by spammers at one point. So google stopped counting URLs completely.
That said, I don't think that "postname" is the wrong way to go - because it helps your VISITORS know better where they are, and it just looks prettier. However using
/%category%/%postname%/ is the best way to go - over using just postname. It allows for URL shortening by the user (when they want to see all the posts in a category, they simply delete the part after the last slash - yes there are LOTS of people who do that!), and further clarifies where your content goes to the search engines. There are also documented problems with just using
postname and nothing else:
"Note on using only %postname%
If you use postname as the only element in your permalinks to create a structure such as myblog.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. 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."
found that here: http://codex.wordpress.org/Using_Permalinks#Structure_Tags