Itqualityassurance
It's all up to aesthetics really; since app icons all function the same (except for Folders, which have a border to suggest that they are folders) there's really no reason to add or not add a border.
What is the size of an Apple app icon?
As for iOS, app icons should be sized at 1024×1024 pixels. Similar to Android, the icon will be resized depending on the device and context, but Apple takes care of that for you.
How an app icon should be?
An icon can be detailed or simple; just make sure it's creative and interesting and accurately conveys your intentions. Your icon will be vying for attention among thousands of other icons, all of which have the same 1024-pixel canvas to make their impact and secure their connection with the viewer.
