I'd rather step up starters from one vial. Saves cash. Check this out(from BN forum):
"You can tweak the calculator at Mr Malty to help you work out starter stepping up.
Say you want are brewing a 1.072 bock and you want 500billion cells. Thats either a 6L starter on a stirplate, or 5 fresh packs of yeast. But you can step up instead. You are going to pitch a fresh tube/smackpack into 1.0L of wort as your first step.
So what you do, is juggle the gravity and volume numbers in the calculator, till it tells you that you need a 1.01L starter - dont worry about what you are actually brewing, just muck about till it tells you what you need is 1.01L. Now look at the amount of cells it says that will be in that 1.01L starter. Remember that number. (its about 237billion cells)
OK, without changing anything else - go to the % viability input box and increase it. It will go up past 100%. Increase it till the line that says "vials or packs needed without starter" decreases to 1. It will be roughly the amount of cells you have divided by 100billion. so for our example, the percentage figure where the packs needed figure changes from 1.1 to 1.0 is 226% and it says no starter required.
so effectively you have one smackpack with 226% viability.
Go back to the start - plug in the gravity and volume of the beer you want to make - and set the viability to the percentage figure you just worked out (226%) - and now the calculator will tell you what size to make your "second" starter if you use the yeast from the first one. in this example it tells us we need a 2L starter to go from our 237billion cells to 500billion.
So you start with a fresh vial - make a 1L starter - then either decant the beer and add 2L of fresh wort to the yeast, OR assume that all the nutrient is gone, and add a further 1L of double strength wort (which when diluted with the first one = 2L of starter strength wort) - and this will give you the approximately 500billion cells you need. And your starter was never more than 2L.
Ive gone 1L - 2L - 4L and ended up with over 1000billion cells for a big batch of strong beer."
edit: quotes and link