editing files via sftp - permissions, users and groups...

i'm reasonably well setup now (for a linux bodger) with a ubuntu 10.04 linode, with an apache2 based lamp server.

however i'm a bit confused as to best practice re: users, groups and permissions: all files on the webserver are owned by www-data. i login via sftp to the server as USERX (i.e. not root) who is a member of www-data group. but that user cannot write files unless i change the permissions of all folders to include group write. do i really have to change all files and folders from the default permissions, or am I missing something blindingly obvious? i assume the latter…

thanks a lot