From what I understand per osha regulations any pressurized cylinder is supposed to be tested and certified every 5 or 7 years or so. At work we have to have all of our lpg tanks and air compressor tanks recertified on a regular basis. Same goes for no2, oxygen, acetelyne, gold gas, etc. Basicly ANY pressurized cylinder. You suba divers out there should be aware of this all ready. The people refilling the tanks are supposed to check the stamp and only fill it if its current. This goes for steel tanks, carbon fiber tanks have a longer lifespan but I don't think they can be recertified they have to be replaced at the end of their certification period.
They do this because after so many fill/discharge cycles and expansion/contraction due to weather the tank devolops stress fractures and will eventually fail regardless of wether or not the pressure relief valve is good.
In short check your tank. If its steel and over 7 years old have it certified.