What is ARC?
In Objective-C and Swift programming, Automatic Reference Counting (ARC) is a memory management enhancement where the burden of keeping track of an object's reference count is lifted from the programmer to the compiler. In traditional Objective-C, the programmer would send retain and release messages to objects in order to mark objects for deallocation or to prevent deallocation. Under ARC, the compiler does this automatically by examining the source code and then adding the retain and release messages in the compiled code.
How should I use ARC in tweaks?
As ARC was first introduced with iOS 5, it is very likely that iOS utilizes manual memory management (citation needed), as it seems unlikely that they would rewrite their entire codebase to support ARC. For this reason, it is not recommended that you use ARC in any hooks, as it could potentially cause problems -- injecting automatically managed-memory code into a manually memory-managed system simply doesn't sound like a good idea.
However, you probably will not encounter any problems using ARC in any custom classes that you create.