According to Apple\'s ARC Documentation, there are a fairly significant number of changes to the way that one develops software when using ARC.
As a complete beginne
It would be a great idea just to go over it and understand what's going on, but for development it's not essential, and as you can see most if not all developers have moved their deployment targets to iOS 5.0+, so you are likely not to develop under manual reference counting.
However if you plan to use non ROP -retainable object pointers- in your code like CFStringRef, you might want to really look at non ARC so you can understand things like bridge, because you can combine ARC and non ARC code in one project.