I struggle a lot with that one, I hate my hesitation time on that topic when coding.

When I think about it, it feels like self variables should be avoided here are some reasons (from my perspective):

• Closer to a black box approach where inputs and outputs are clearly defined.
• Self variables are often left aside when defining method documentation, this can hide the impact of the inputs and the resulting outputs.
• (I like this one) Methods wich avoid the usage of self variables are easier to move outside the class. Removing a method from your class often simplify testing cases.

That being said, in some situation (even if you should normally avoid it) you might be a situation where many inputs are required by your method, at some point passing arguments starts to feel ridiculous.

In conclusion, try to apply a rule you follow or you will be stuck in a mixed approached like me which leads to dirty code :s