There’s a section on WordPress file permissions in the Codex article on Hardening WordPress. The article doesn’t talk much about Multisite, but it should be treated similarly to the rest of wp-content — ie, the server process needs to be able to write to it.
I’d steer clear of doing chown www-data:www-data [/path/]wp-content/uploads/sites — you should be able to achieve the same effect by using chgrp, which changes the group permissions on the files & directories. Something like
chgrp -R www-data wp-content/uploads/sites
chmod -R g+w wp-content/uploads/sites
might be what you’re looking for.
The first line changes the group that owns the directory in question (and the -R applies it to all files and subdirectories too). The second line changes the “mode” (the permissions) so that the group can write files and directories. (-R again does it recursively; g+w is the group-writeable part.)
Caveat — every setup is different; you should probably double-check with your hosting company if this setup is kosher.