On most systems it doesn't matter what the numeric user and group ids are, but if you use the Network filesystem (NFS), you need to have the same uid and gid on all systems. This is because NFS also identifies users with the numeric uids. If you aren't using NFS, you can let your account creation tool pick them automatically.
If you are using NFS, you'll have to be invent a mechanism for synchronizing account information. One alternative is to the NIS system (see [Kir]).
META: this is wrong place? However, you should try to avoid re-using numeric uid's (and textual usernames), because the new owner of the uid (or username) may get access to the old owner's files (or mail, or whatever).