In my experience, there's potentially more overhead later on, if you only grant update privileges to a single user.
Therefore, I recommend you go ahead and setup a User Group (Tools | User Groups...), even if it only has ONE user.
Then, set the Update Rights for that field to that one group. You do this by right-clicking on the field in question and selecting Properties. You then go to the Security tab and set the Update Rights drop-down to the group you just created.
Hope this feedback helps.