I would agree that a yeast strain has a certain characteristic, however once you put the yeast in a less then ideal situation, different PH, OH concentration it will stress the yeast and lead to mutations that are selective for the current conditions. This is one reason why i always save my...
Wouldn't the best way to keep a cell line be, to create a large starter let is settle a bit, decant as much liquid off the top, save some of this for next time in a soda preform and keep the rest to pitch in your wort, if you still need some extra cells, add a little wort back into the starter...
I live in Quincy and go to Gorham's in West quincy
http://www.gorhamfire.com
I find them to be way cheaper then the welding supply place i have gone to, 5 lb is 10 maybe 15$ i alway get a 5 lb and 15# tank filed and its 25$
I buy soda preform bottles (same as white labs) I then wash the yeast down and decant most of the water off and pour with a funnel into the preforms, then i made a shelf in my fridge that has holes in it rough 27 so i can keep plenty of yeast around...