My name is Cassie, it's not short for Cassandra, simply Cassie on my birth certificate... I personally can't stand the name Cassandra, however I'm constantly being asked if it is short for Cassandra.
To answer your question about nicknames, I never had anyone call me Assie or Ass... the most people would come up with is Cassie Wassie, which was harmless and I never felt teased by it, if anything I found it to be an endearing nickname because it was only people I cared about who used to call me that whenever we were being playful (it was only when I was young too)! People closest to me call me Cass though, and only my parents and people who just meet me tend to call me Cassie. I love my name and I love even more that I didn't have a million Cassie's or Cassandra's being called Cassie in school growing up. To this day I've only met 2 other Cassie's, one like me is only named Cassie & the other insists on being called Cassie, not Cassandra.
I think it's safe to say that your child would never get teased for their name, because no-one ever found one insulting enough for me or the two other girls I know. They agree they get called Cass & received their endearing playing name of Cassie Wassie, which was never seen as anything more than a loving playful nickname only said during those playful moments... one even uses "CassieWassie" for her Pinterest page... so you're safe with this name.
As for Sandy/Sandra... I've never met or known a Cassandra or Cassie who received these nicknames... so I highly doubt they would become a nickname unless you called her that yourself and introduced her that way to others. I would say that if this is the name you choose, your child will decide on the name she likes best... my parents didn't like Cassandra, preferred Cassie & when I was young, they used to correct people if they called me either Cassandra or Cass... now, I've had to tell my parents that I love being called Cass, to me it's an affectionate name and I LOVE it, as I love my name Cassie!
I will say though, that being Australian, we are known for our nicknames and slang and I had so many close friends come up with crazy different nicknames that I didn't mind either... for example: Cassaroo, Cassaroony, Casserole, Cassio etc... These all made me laugh though and I never had an issue answering to them. Call me Cassandra though, and I'll flat out ignore you! lol - also just to clarify, my membership name says Cashaz, but that is only because my middle name is Sharon!