Ummm, no.
Not "any" aluminum cylinder made before 1990 is suspect, and 6351-T6 alloy is not necessarily "bad". Shops that refuse to fill "any" cylinder manufactured prior to 1990 are employing a Draconian measure based on ignorance, non-rational fear, greed, or all the above.
First, please take the time to carefully read the actual DOT statement regarding these tanks, which is more current (and more authoritative) than the article linked above.
http://www.gpo.gov/fdsys/pkg/FR-2006-08-29/pdf/E6-14255.pdf
To summarize, there
have been incidents of these tanks rupturing during filling (but nobody seems to worry about steel tanks, which have
more reported ruptures).
The story on these ruptures is not cut and dry. There is some evidence that the tanks that ruptured at least in some cases had been overfilled in the past, and some had apparently been used in Florida cave country, where routine significant
overfilling of
LP Steel tanks is common; but significant overfilling of
Aluminum tanks is
not ok under any circumstances.
Neck cracking in some cases was also thought to be caused by over-torqueing the tank valve after VIP by a ham-fisted shop employee, causing cracks in the threads.
The final word from the DOT is that these tanks are fine to continue using, but they need to pass hydro and visual eddy testing. The visual inspection cannot be hap-hazard as may be the case with some shops.
Yes, I have a VERY old (1978) Aqualung Al80, yes it is the "bad" alloy, and yes I will happily continue to dive it for just as long as it passes inspection.... and my dive shop owner is also an "old timer", and has no issues with the "bad" alloy as long as they pass his inspection and hydro.
This article by PSI in 2011 is also worth a quick read:
http://www.psicylinders.com/inspectors/library/18-a-short-review-of-6351-alloy-aluminum-cylinders
Having said all that, there
are shops who will not fill these tanks, even if they pass hydro and visual eddy testing. I agree that is their right, but have a suspicion that in some cases they want to sell you a new tank.
Best wishes.