+1 The yeast you get from Wyeast and white labs was generated with a much bigger starter than you get in the little packet, they have crash cooled and decanted the spent starter wort. If you want to figure out the max step size from your packet it is a little complicated.
1) figure out how many viable yeast cells you have, you can punch your production date into Mr Malty or do some research on yeast viability from production. Remember this is a guess as heat in transit can affect your yeast.
2) work out how big a starter you would need to make to generate the amount of yeast you calculated that you have from step 1 using your process, ie stir plate or shake with O2 etc etc. Take this number as your previous step and multiple that by your step rate to get your next step.
Note some brewing schools (German I think) teach x4 steps other teach x8, x10 is considered the maximum step. I do x5 steps in mine when I'm working from slants it seams to work well. The theory behind step size is, too small and the yeast will get lazy, too big and bacteria and all those other nasties have a better chance of eating the starter than your yeast. Also if you don't have a stir plate smaller steps are better as the amount of O2 you can get into your starter is greatly reduced so when the yeast run out of O2 they will just make alcohol instead of babies.
Hope this helps