Originally Posted by Mcsuck
3% agar is probably also just a waste of agar. I work in a yeast lab, and all of our plates are made with 2% or less agar. I would be wary about using year old yeast from the fridge because of mutation, especially if you are picking single colonies (greater chance your mutation will proliferate). I would say a few months would be ok, much better to freeze down and make a yeast library that you can collect from though.
I think a lot of people here recommend processing the whole slant rather than picking an individual colony for this very reason. I have been making my 20% glycerol stocks in 1ml aliquots, and just using the whole thing to innoculate.
I'm in the same field as you, I work on directed evolution in yeast, and not going from single colony is foreign to me! It makes sense though, because having mixed background allows competition to do its thing.