If you are in a position to ask this question I humbly recommend you default to: “No, I do not need this as an instance variable right now.” There is a tendency to plan for every potentiality when doing software development and this is a prime example of such a situation. Resist the temptation.
Instance variables represent state associated with an object in memory. When you declare an instance variable you are saying to all future developers: “This piece of state needs to last for the life time of this object because it will be used elsewhere.” Think very hard about that. We are saying loudly here that this variable is coupled to other fragments of code. Which code? Dunno, do a global search ;).
This is called tight coupling. I will go so far to say that coupling is the source of all evil and complexity that we deal with as programmers. With every step that we, as programmers, take in building a system should be with an eye to reduce or at least not increase the coupling in the system that we are working on. The fewer instance variables that we have in a Rails controller the smaller the potential coupling between the view and other method calls that have access to those instance variables. Each method, each action in a controller, should be as self contained as possible. The less leakage of state out of a function the better. Generally your view doesn’t need to see that temp variable of the calculations that you made to set a model attribute keep it local, drop the “@”. Worst case scenario you go back and refactor your code to include that “@”.