A good yogurt will contain at least 5 or 6 different species of bacteria, not just acidophillus. If you use a satrter culture with all of these, then use your culture for the next bath, at some point down the road, one or more species may be reduced. For this reason, you might want to use only 3 or 4 successive cultures, then use a new starter. There is also an increased risk of contamination over time which may allow unwanted bacteria into the culture.

Another way is to get cultures of individual bacteria species, culture each, then blend them together. This way, you know that you will get more consistant product.