A pet peeve of mine, is when I have signed an email as Nikki and the person writes back Nicole.
I had someone email me via Kijiji, regarding something I am selling. I responded to their question and signed "Nikki". When she wrote back, she used Nicole.
A) How do you know Nikki is short for Nicole? My name could just BE Nikki. B) My name IS short for Nicole, but when I sign as Nikki, you call me Nikki.
Just as annoying would be if I signed as Nicole and they responded Nikki. If you don't know someone, you should address them as the name they have introduced themselves as, not as whatever you feel is comfortable. That is reserved for friends and family members.