What criteria do you use when deciding to create a new webID? Since they allow admin permissions and access to resource/log reporting by domain, I usually do the following. In addition to my master webID, I create a webID for every domain that needs to access reports, and restrict the permissions.
Sometimes webIDs might need to access multiple domains, if I resell hosting to a business, but there has never been a need for anymore than that one additional account per domain. Since you can restrict disk space by actual user account, and bandwidth usage by domain, this probably gives you enough control.