Yeast growth is affected by inoculation rate. I don't have the book Yeast in front of me right now, but there is a sweet spot of cells per ml that results in maximal growth. 1 gallon would be a pretty low inoculation rate for that viability. The sweet spot is about 1.5L to 4L for 100b cells, IIRC. If you're at 33% viability, multiply those volumes by 1/3 to get the optimal growth range (0.5L - 1.32L).
So to answer your question directly, ideally a stepped starter would be used to keep the inoculation rate in the sweet spot as much as is feasible. Let's say you have 33b cells, and they grow 100% in a 1L starter. Now you have 66b cells. The new "sweet spot' is 1L - 2.64L. Let's say they grow another 100% in a 2L starter. Now you have 132b cells. The new "sweet spot" is 2L - 5L, and so on.
If you're anything like me, once you crunch all the numbers to get a reasonable pitching rate, you'll quickly see the benefits of using dried yeast (20b cells per gram ~ 230b per pack, with reliably 90% viability).