I don't think there's anything wrong with telling a woman you think she's beautiful, as long as you're being genuine of course. Although, I think you're taking a chance of being belittled doing it in DC with the majority of them being nasty bitches around here.
There was a woman in the Fresh Fields in Rockville a few months ago and she caught me staring at her, so I said, "I'm sorry for staring, but you have the most beautiful eyes" She thanked me for the compliment and we went our seperate ways. I decided to be honest because that was the reason she caught my eye.