What should I be called for my nickname?

My name is Mathieu with 1 t and I have always been called matt with 2 ts but lately I have been wondering if my nickname should be Mat instead of Matt. What do you think?

I think it's your name and if you want one T then you spell your nickname with one T. But, usually (when I think of nicknames) nicknames are said allowed, and Mat and Matt sound the same to me. So, does it matter?