It's a sliding scale - you are very unlikely to have a perfectly pure culture at any point, including at arrival, so it's all about managing the contaminants. If there's a little film algae, maybe you can outpace its growth by using shorter grow cycles and switching to a new container more frequently. Maybe you're seeing a small amount of your algae build up on surfaces because the aeration/flow level could be higher. Maybe you've still got good production and would for a while, but the annoyance of cleaning up the film algae is enough for you to want to restart.
I think, generally, that resetting a culture from a good source when there's contamination is the right call in the long term, it can get tedious and expensive, so getting your culture to keep going with some contamination present may be a more practical solution until, for example, another starts to have an issue and then you don't have to pay for shipping twice for the fresh starter cultures.
