no you don't HAVE to update the FS and custpack if you are just going from a generic FW to a newer generic FW. I would recommend it but you don't have to.

The only time you will need to "customize" is if you flash the file system.. If you just flash the main firmware the FS stays un changed.

If you want to change the language you are going to have to flash all 3 things. MAIN,FS,and then customize w/ a pack that matches that FS.