What is ARC?
ARC (or automatic reference counting), is a new method of garbage collection in Objective-C, first fully introduced in 2011 with iOS 5. ARC serves as an alternative to Objective-C's historic garbage collection model, in which memory is managed manually.
While ARC can be very convenient, it can also be used as a crutch. ARC is a helpful tool while learning as one does not have to learn how garbage collection works along with the rest of the language. However, it is important not to rely on it, and to still be capable of managing memory manually, as ARC will not be available in all scenarios.
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.