When do I sign my application?
If you are filling out a DS-11 you can only sign the letter when instructed to do so by the Acceptance Agent at the time of execution. Please do not sign it beforehand or else you will have to fill out another application.
For all other forms applications (DS-82, 5504, etc.) you can sign them at any time. They do not have to be signed in front of any specific government agent.