no, it's not necessarily needed to get authentication to work, but it's just to keep everything matching up.
what I forgot to ask (and stupidly assumed that the answer was yes) was do you have your users/groups set up on the LDAP server, and are your databases set to authenticate from a remote server?
for example: to authenticate from a remote (OS X) server using LDAP, you need to have a user "Admin" (which you can create with the Workgroup Manager).
I would recommend keeping the user "Admin" though just authenticated via filemaker, then make a user for yourself, like "Joe Blow", put joe blow in the Filemaker Administrators group (call it whatever you want, but you'll have to make the group), then under Define Database in Filemaker, make a new account, authenticated from the remote server with an access level of Full Access, then type in the group name that you want to have administrator access, then you can just log in to that database with your own username/password of Joe Blow.
-rich