In my twenty-five years of helping people in customer service positions, reading, and talking with people, I have yet to come across an answer, or even an attempt at an answer. It's kind of a mystery to come across people who believe customer service representatives can somehow escape human Psychology.
No Question Smiling At Customers Is Good
There's no argument with this. It's an instance where common sense and Psychology actually agree with each other. At least provided smiling fits the context and mood of the customer -- in other words, you wouldn't want to smile while a customer is expounding on his hardships, or is angry, or let's say, is talking about a death in the family. You know that part.
Genuineness - Misleading In Customer Service Situations
Here's the problem. Genuineness means accurately portraying your REAL mood. So what happens if you have a family issue, or for whatever reason, feel unhappy and miserably. It's human, it happens a fair amount, ande there are very few people -- perhaps NO people, who are happy all the time.
To be genuine means that you don't pretend to be other than you are. Clearly this isn't a good idea when you feel sad, or really are not looking forward to interactions with your customers. To be genuine within this context means NOT faking a smile, not putting on some fake front, and most customers don't want to know, and don't care what's going on with you mood-wise or life-wise. What customer wants to interact with a "sad sack?" Why? Because it will appear to the customer that you aren't interested in him or her, and truth to tell, sometimes you aren't interested.
So, What To Do With Genuineness and Smiling?
Smiling IS good when you feel it. Fake smiles a lot less so, because, as we learn more about facial expressions, it seems like fake smiles use different micro-muscles in the race, and people seem to be able to read those signals on a sub-conscious level. In other words, they'll know.
So, avoid the fake smiles, and simply do your best to be polite, helpful and all those other good things. Try to speak with a reasonable amount of animation, and you'll do fine.
As for being genuine, my advice would be to forget about it as a rule. Perhaps someone else will comment on how it fits into the reality of customer service, but frankly, I don't think it's important. And, I have no idea why people preach it.