If it hasn't been an issue with getting them filled, don't worry about it. My two forklift tanks are waaaay out of date and I've had no problems.
I could see it being an issue if you went to a distributor, or had them filled on site, as that could be a cya issue for them if something went pear shaped, but just going to Corner Gas where they have a propane station in the back corner, they probably don't even know about certifications.
I am rather surprised to hear that some distributors will fill 40 yo tanks, but that could be regional, long time good customer relationship, lots of factors.