Just focusing on the "If it is an issue, what's the best way to deal with it?"
There are lots of NCOA list cleaning services that will check the address and replace stale addresses with new addresses IF the recipient has mail-forwarding enabled with USPS. If you're shipping in-house, just do a search for NCOA list cleaning and get 3 or 4 quotes.
If you're using a logistics vendor, they should be able to do this for you.
The cost will run anywhere from $0,10 per address (for tiny lists) down to well below $0,01 per address. Most list cleaners will charge some minimum fee (e.g. I think Experian charges $50). The big vendors are MelissaData, Experian etc. but you should definitely ping lots of small mail shops. Your local direct mail vendor may be willing to do it for very little!