Don't do it.
I don't care how casual you guys feel around each other.
I don't care how well you get along.
I don't care how natural it comes.
Just don't do it.
Why?
3 reasons.
Reason #1: She is and most likely always will be your superior in the workplace, and she will always have to address you as an employee. When things go wrong those touchy feelings get in the way. This makes things a lot more complicated.
Reason #2: People notice no matter how much they try and hide it. Eventually they will start to assume you're sleeping with her. This leads to jealousy because any awards or accolades given to you at this point will be seen as "kiss ass" treatment for your relationship with the manager.
Reason #3: If anything goes wrong, and it will,Trust me, you still have to work with this person. Every. Single. Day. This is extra stress you just don't want to put yourself under.
Take it from me guys, keep things as professional as possible.
Unless of course you plan on quitting the next day.
Then she's fair game.