Can NOS Capacitors be trusted?

I have a box of over 100 NOS Sprague, Mallory & General Instrument FT & Twist-Lock Mulit-Section caps i've been hanging on to for years. My question is, even with proper "Reforming" can parts that old be trusted for the long term?
Depends on how much you've got to lose if they go bad. For a home stereo system, they might be worth a try . . . but for a working musician's guitar amp, definately a bad idea. I restored a lot of tube gear in the early-1990s using NOS twist-locs like the ones you describe, and the failure rate within the first year after service was at least 25%. The ones that did last a year or two seemed to be pretty solid after that, however.

But that was at a time when there were virtually no newly-manufactured twist-locs on the market, so it was worth playing around with them (especially if you're anal about originality). Today however, there are many common configurations of new twist-locs available, so for i.e. a Dynaco, there's no reason not to use a new one. Because no matter how careful you are about "reforming" procedures, old electrolytics are a crap shoot. (Actually, that seems to apply to electrolytics in general.)
Just so you know Kirkus the new CE caps for the Dynaco are awful. They are only rated at 55 deg. C and get overheated easily by the 5AR4 that stands next to it. I have tried them repeatedly and they seem to fail after about a year. There are now circuit boards being made with single section radial caps built up for the multi sections that are rated at a much higher ambient temperature 105 deg. C that work great. Generally speaking twist locks were awful caps even when they were new. I generally avoid them as much as possible. If you have some radial caps or axial caps and the rubber gaskets on the + side aren't cracked and you can reform them and test their leakage current, and its low, their probably safe to use. That being said caution is advised.
It's been over 12 years ago, but when I needed the multi-section cap for a Dynaco I was restoring, these guys had just gotten some fresh ones manufactured for them, to original specs: ( I gave the(by then well modded) ST-70 to my son a few years back, still working wonderfully. Try calling them: (800.706.6789)