I believe that when you stamp and sign a document you have something akin to a trademark. This can be important in the future because I don’t believe that anyone or firm could stop you from using that work as part of your portfolio. It may also carry more weight with a future employer to see plans stamped by you in that portfolio.
A solid body of built work that is clearly your own is one of the strongest influences on potential clients in choosing a designer. Anything and everything that works in that direction should be done if you ever plan on not being an employee for your entire career.
That is a great opportunity your employer is giving you. Embrace it.