That would depends on the situation.
If you were an employee and it was a deaf customer having communication problems with a hearing customer, then it's likely to be personal between the two and not for employees to get involved with.
However, if it was something regarding an item in the store (wherever you work at)... then you could "jump in" as you would with every other customer. Just don't make it obvious that you are intervening because the person is deaf. For instance, the deaf customer is asking the hearing customer for opinion on a shirt he/she wants to buy... but can't understand what the hearing person is saying. You could approach both customers equally and ask verbally, "Hello, how are you two today? Can I help you with anyting?" If the deaf person gestures in a manner that he/she is having a hard time communicating, then you can start signing the best you can. This will likely surprise the customer in a good way and realize that he/she can be assisted.
I've done the same with my previous job at a local video rental store. Even though I'm hard-of-hearing, I've always talked like a hearing person with everyone. There was a deaf guy who approached the front counter to check out his movies. I didn't know he was deaf at first and I said, "Hello, you find everything alright?" He just kept his eyes down... placed his membership card, ID, and movies on the counter... and kept looking up and down as if he was shy. When I tried to get his attention, he gestured that he was deaf. So, I signed out how much he owed. When he saw me sign, his eyes opened wide with awe... and smiled. From that point on, he always looked for me whenever he entered the store knowing that there was someone who could assist him whenever necessary.