As with most implementation and reporting in AA, I normally start off with business requirements. In this particular case, I would consider what are the obstacles that prevent users from converting.
With that thinking, I normally narrow down my error reporting to be based on the errors that prevented a user from continuing his journey. It doesn't have to be the JavaScript error (though most times, it is) nor a system error number-and-message. It just needs to be a "reason" that can fit within the 255-character limit of an eVar. Knowing the error "reason" together with its timestamp is normally sufficient to start investigating with the developers.
I would normally configure that error eVar to have a "Hit" expiry. Again, from the user's perspective, I would assume that encountering an error is a momentary irritation, such that the user is likely to try doing what he was attempting earlier. In that case, there's little reason to expire the error much later after it has occurred. If I need to know what the user did after the error occurred, I can use a sequential segment to find that out.
Hope that helps with your research.