Wow, it's funny that I should run into this topic. I'm at this moment listening to my favorite song about the subject, called "Come" (come to think of it, it's the only song I know of about it...)
It wasn't part of my first experience by any means, but I took up the habit soon after and have come to love it. I wouldn't say it's exactly your duty per se, but if you can develop a healthy willingness to go for it, all in the name of making sure she's as happy as you can make her, then the benefits will be big for the relationship.
It should be mentioned, though, that, in my experience, not every woman wants or enjoys oral stimulation, but if she's interested, maybe you should try to be as well.
Plus (and I'm not saying this is the reason to do it), if you're unwilling to offer her oral pleasure, you can't expect her to reciprocate, you know?
As the song says...
"When I lay U down I'm gonna tell U what 2 feel
Don't be surprised if I make U my daily meal
Lickin' U inside, outside
All sides, up and down
With my tongue in the crease, baby I go 'round
When I go down, down, down, (Come)"
If you can eroticize the experience for yourself and really look for the signals that she's enjoying herself, I think you'll find yourself coming to love it.
Enjoy!