If these are public and do not require users to authenticate to the directory, do not put them in the domain. However, if you need some kind of authentication or looking up information from AD, you might consider running Active Directory Application Mode (ADAM) in the DMZ. You may need to copy the relevant information from AD to the application partition, as ADAM does not synchronize standard AD partitions.
Of course, if the web server is on the same network as the domain controller, it can be added to the domain to increase manageability.