My name is Cynthia. I'd say at least 80% of the people I know call me Cynth. My signature has devolved into this, and I was tempted to put it on my graduation application. (Then my name would have appeared this way on my diploma.) I don't care if you call me Cynth or Cynthia. Both are acceptable. Hell, I'll even answer to Cynthtastic.
Only people who aren't paying any attention call me Cindy. This is presumptuous and rude. You just can't give someone a nickname in which she never expressed any interest. The most audacious of all are the people who correct me mid-handshake. "Nice to meet you. I'm Cynth(ia)." "Cindy?" No. No, that's not what I said. Other names I didn't say include Nancy and Hildegaard. Keep up.
I'm very careful to reply to people -- in person and in e-mail -- by the names they've called themselves or by the names by which they've been introduced to me. I've been caught calling people the wrong name on occasion, but it's usually because the person who introduced us mucked it up.
E-mail is a special case. If my name is WRITTEN OUT for you, get it right. If I sign something "Cynthia," don't reply, "Thanks, Cindy!" What the hell is that? I worked with a Jacquie. I never, ever wrote, "Thanks, Jackie!" Colleagues did it all the time. WHY?!
If I'm calling you a thing you don't like, please tell me. If you're anti-nickname, I will call you by your full name, no matter what it is. If I'm spelling something wrong, I want to know. Please.