When do I need getInput() in a validator and when do I not need it?

[Andrew Coutermarsh]

We're in the process of having to rewrite a large portion of our validator scripts to use getInput() instead of simply referencing the fields, and it's making me wonder exactly when I need to use it. An example:

// Ensure that issues with an SLA Violation have a SLA Violation Reason specified
// Implemented for Translations but seems relevant for all issue types

if (
    isCustomFieldInContext("SLA Violation", project, issueType) &&
    isCustomFieldInContext("SLA Violation Reason", project, issueType) &&
    #{SLA Violation} != "" &&
    #{SLA Violation Reason} == ""
    )
{
    return false,"SLA Violation Reason must be specified";
}

return true;

I know that with those last two conditions I need to change them, e.g.:

if (getInput("SLA Violation"))

But do I need to do it for something like this?

if (resolution == "Done")

 Or any number of other types of built-in fields like Assignee, or Project, or issueType?

Essentially, I'm trying to determine when I can simply reference fields or variables without having to pull that info with a function. Is there a guide somewhere?