It's hugely important! How would you feel if some one came up to you and said you were cute but looked like the opposite sex. Also don't answer that as I know most of you will roll with it.
Don't assume their sex?
I find it hard to instantly label someone's gender based on their looks since I've met plenty of people who don't really fit a general gender descriptor. On top of that as a child I was often mistaken for a girl due to my hair and chubby face and after being so annoyed by it I can't stand the idea of putting someone else through that.
"You are cute"
is not "You're cute as a guy." or "You're cute as a girl."
It's just "You're a cute thing/person/whatever."
If I already know a gender identity and I'm super interested? Sure, I'll be like Aye, you're a cute girl! Let me go blush over here in the corner because reasons!