is the letter a must or optional?

I, h1b holder, plan to visit Canada for one week. I will bring original H1B approval notice, valid paaport, copy of H1B petition package, recent pay stubs, etc. Is the employer letter to show I am employed and will back to work before certain date a must or just optional? It is better to have it, but without it I am going to have a trouble to come back, legally? Thanks.