Yes, absolutely.

1. Many (most) regionals require that you have the written done to interview.
2. Many (most) regionals will give you a written test containing select questions from the 121 ATP test.
4. The knowledge will help you some in indoc class.
3. When you finally get awarded an upgrade you will probably be too busy with life to be dealing with the written before class starts.

But take the 121 version, not 135 unless you are planning a career in charter.