[name_f]My[/name_f] husband and I love the name [name_u]Cassidy[/name_u] for our girl. I just have one reservation: I do NOT want people shortening her name to [name_f]Cassie[/name_f]. no offense to all the [name_f]Cassie[/name_f]'s out there, but I really really don’t like that name. [name_u]Cass[/name_u] would be a cute nickname. So my question is, how often do you think people will do this? lol
I have a girl in my class at work named [name_u]Cassidy[/name_u]. I never, ever call her [name_f]Cassie[/name_f].
I think [name_u]Cass[/name_u] as a short form might pop up from time to time (goes for any short form, really), but I think the general public won’t try to give cutesy nicknames to people unless they say that’s what they go by.
Like if your name is [name_f]Kristen[/name_f], people in your life will probably call you [name_u]Kris[/name_u] once in a while to save time or syllables or whatever. Few people you meet will be so bold to make the jump to something personal/ “cutesy” like [name_f]Kristy[/name_f] Or [name_f]Krissy[/name_f] unless that’s what you say your name is.
If you don’t call your [name_u]Cassidy[/name_u] “[name_f]Cassie[/name_f]” I think the vast majority of people you or she comes across won’t do it either (unless she decides she prefer [name_f]Cassie[/name_f]). It isn’t completely unavoidable though- someone [name_m]WILL[/name_m] call her [name_f]Cassie[/name_f] at least once in her life 
They shouldn’t if you kindly, but firmly, point out that its [name_u]Cassidy[/name_u] not [name_f]Cassie[/name_f].
I have a cousin named [name_u]Cassidy[/name_u]. She goes by [name_u]Cassidy[/name_u] 90% of the time, with her twin brother and big sister sometimes calling her [name_u]Cass[/name_u]. I heard someone call her [name_f]Cassie[/name_f] once and she turned and glared at them (she’s 8) and said, “You can call me [name_u]Cassidy[/name_u] or [name_u]Cass[/name_u] but you CAN’T call me [name_f]Cassie[/name_f].” Hahaha we all laughed. But you know, if you name her [name_u]Cassidy[/name_u], there will come a day when she might decide she wants to be called [name_f]Cassie[/name_f]. Eventually there’s a point when parents have no control over their kids names.
Another anecdote: [name_f]My[/name_f] brother’s name is [name_m]William[/name_m]. When he was little, my mom called him [name_u]Billy[/name_u]. He stopped her and said, “You can call me [name_m]William[/name_m], Sweet [name_m]William[/name_m] or [name_m]Will[/name_m] (he’d never been called [name_m]Will[/name_m] in his life, he just made it up himself). But I don’t want to be called [name_u]Billy[/name_u] anymore.” He was only 2 or 3.
So I would suggest only picking a name if you love the name and all (or almost all) the nicknames that come with it.
I think if you call [name_u]Cassidy[/name_u] [name_u]Cassidy[/name_u] with the occasional “cass” thrown in for convenience like “hey [name_u]Cass[/name_u], time to go!” then the odds are she and everyone else will go with [name_u]Cassidy[/name_u] as well. [name_u]Waverly[/name_u] makes a good point about the chance that SHE may choose to go by [name_f]Cassie[/name_f] but honestly, I think if you call her [name_u]Cassidy[/name_u] then that will be tied to her identity and it’ll stick.