How far out of balance can a Lipo get befor I should worry ?

[Bondonutz]

I checked on one of my E-Revos today that I hadn't run in a month or so. I couldn't remember what state the Lipos were in since I used it last, anyway looks like I put it up with fully charged 2s packs (TrueRC 15C -20C burst)
I've had these packs for nearly 2 yrs and have served my very well so if they are on the way out I'm fine w/that.

One pack is even across the board and the other the #1 cell is 0.5 volts lower than #2. It stays this way regardless using the DUO3 to discharge or running them down via driving and when charging it falls 0.4-0.5 short still. The pack is still hard as a brick, no puffiness at all.

I never had a Lipo start to go like this, they usually start to swell or a cell just falls completely. Should I be prepared to chuck it or just run the dam thing till it starts to puff or not worry at all ?

EDIT, Now that I have a DOU3 I plan on doing the "storage" thing, Is 80% still the prefered percentage to doing this ?