In object-oriented terms, does the variable make sense as an attribute of the object? If so, you should make it a field variable. If not, it can go either way.
Remember the Single Responsibility Principle -- well-designed classes should have only 1 responsibility, and thus only 1 reason to change.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…