Hi Zakk!
That's an OK strategy. Here one annoying thing with that:
One issue with that approach is new files created in code (cache, logs, etc) won't create new files as group-writable. This means that user wwwuser, while part of group www-data, won't necessarily be able to edit those files.
One way around that is to use ACL's instead of just regular linux permissions, as these let you set defaults for new files/directories.
Optionally
I'm assuming you created user wwwuser as someone you can log in with, perhaps to deploy? If that's a system user who cannot login, I'd just keep using www-data as the user for application files. However, assuming you want to keep using wwwuser to log in with:
Consider adjusting php-fpm (and perhaps Nginx) to run as user wwwuser and group www-data. That way PHP will run as user/group wwwuser:www-data and you don't have to worry about group permissions and whether new files default to letting you write (g+r permissions).
This is how Forge sets itself up so file write-permissions don't become problematic.
Assuming wwwuser is a user to log in as