I agree with previous posters that this safety issue needs to be handled by your employer, not by asking customers to leave while their meat is sliced.
Besides the fact that it's the employer's responsibility, not the customers', making this request of customers really isn't going to protect you. First, not all customers will cooperate. Second, even if every customer does exactly as instructed, you can still be startled by new customers walking up to the counter, by people walking past the counter, by a coworker dropping something or otherwise making unexpected noise, etc. Your safety should not rely on preventing anything from startling you--instead, you need safety measures that will protect you even when you are startled.