"Copy" when using ARC

"Copy" when using ARC

I know that when using ARC and you have an NSString property, you do
@property(nonatomic, copy) just as you would MRC. But I'm wondering, after
I converted my project to ARC, I still have this in my initializer method:
_someString = [someStringParameter copy]
Is this a bug? Or even with ARC, do I still need to explicitly say "copy"
? Or should I just do:
self.someString = someStringParameter
and all will be OK? Bit confused here...