The most usual rule consists in chmoding the directories to 755 and files to 644.
More and more hosters will assign rights automatically.

These values are accurate in many cases, but have to be adapted according to the security policy of each hoster..

ATTENTION: You are to refer to your hoster's documentation in this respect.
 

For instance:

Folder and sub-folders data 777
Files data/* 666

Note :  /* means that all files of the directory must be chmoded as indicated.
Example :
Files data/* : rights = 666. UNIX command = chmod 666
                  data/* means all files in data directorymust e chmoded in en 666

How do you know that you must apply these "CHMOD" ?

When you get loads of error messages like the following, then you have to chmod::

     Warning: fopen("data/logh.dtb","w") - Permission denied in http://www.mysite.com/inc/functions.php on line xxx depending on the version.

     Warning: Supplied argument is not a valid File-Handle resource in http://www.mysite.com/inc/ functions.php on line xxx depending on the version.

     Warning: fopen("../data/logh.dtb", "w") - Read-only file system in functions.php on line xxx depending on the version.

     Warning: fputs(): supplied argument is not a valid File-Handle resource in functions.php on line xxx depending on the version.