Bobby M recently did a test on year old stored yeast here; http://www.homebrewtalk.com/f13/test...bility-126707/
And my LHBS cells outdated tubes and packs of yeast dirt cheap 2-3 dollars each
and I usually grab a couple tubes of belgian or other interesting yeast when I am there and shove it in my fridge. and I have never had a problem with one of those tubes.
I usually make a starter but I once pitched a year old tube of Belgian High Gravity yeast directly into a 2.5 gallon batch of a Belgian Dark Strong, and after about 4 days it took off beautifully.
Same with jarred yeast.
With any stored, old yeast you just need first to apply the "sniff test" if it smell bad, especially if it smells like week old gorilla poop in a diaper left on the side of the road in the heat of summer.
Then make a starter, and if it takes off you are fine. The purpose of a starter is to reproduce any viable cells in a batch of yeast....that;s how we can grow a starter form the dregs in a bottle of beer incrementally...and that beer may be months old.
Even if you have a few still living cells, you can grow them....That's how we can harvest a huge starter (incrementally) from the dregs in a bottle of some commercial beers. You take those few living cells and grow them into more.
If yeast can be grown from a tiny amount that has been encased in amber for 45 million years, 45 million year old yeast ferments amber ale
we really don't need to sweat too much about yeast viability....
we just need to think in terms of making starters.