Never assign www-data to any group, especially not a group like sudo that can gain admin priviliges. If there's a major security flaw in WordPress or one of it's plugins, there's a chance that the priviliges from the assigned group could be used to compromised the rest of your system.
Don't assign www-data a full 7 for everything, for the same exact reason. Instead, do the follolowing:
Code:
chown -R www-data:sudo /your/web/site
chmod -R 570 /your/web/site/
chmod -R 770 /your/web/site/plugins/
chmod -R 770 /your/web/site/uploads/
chmod -R 670 /your/web/site/uploads/*
Replace the above as necessary. Notice that this assigns the number 5 (read/execute) to www-data for most of your site, and a 7 (read/write/execute) for plugins and uploads directories. The finally chmod allows read/write for the contents of uploads, as a security measure; directories, for some reason, need execute permissions to be accessed, while files don't -- especially not ones being uploaded by your users.
One thing I'd like to point out -- assigning group ownership to sudo seems a bit overly redundant since anyone using sudo to modify stuff can modify anything that root can (which is everything on your server). All that does is allow anyone in the sudo group to modify stuff without using the sudo command. That could be a security risk, if only a minor one provided you can trust your admins.