It's very uncommon in today's world for one of the three factors you mention not to change within a year's time. Depending on how tightly spec'ed your packaging is, it may be worth it for you to retest annually.
Many of my customers depend on the packaging supplier to manage the spec, which can mean you don't know when an "improvement" happens.
More than once, I've found that packaging suppliers make "improvements" to their materials and have only discovered these "improvements" during testing (or when a package that never bothered me before becomes a huge production problem).
However, if they know you are looking, they will usually not make "improvements" without asking.