Depends on the clerk and if there are other possibilities to prove your identity. I once had to fill out a postal identification to prove my new (online) bank that I am the person trying to open an account. The clerk needed to check my identity, I showed my ID and had to sign as authentication, but as I have (like OP) no consistent signature, it took three attempts until it was accepted. But then I think, why do you not look at my ID? There's a picture of me in there!