What is a necessary step after filling a jockey box with cleaner?

Flushing the jockey box with water after filling it with cleaner is crucial for ensuring that any residual cleaning substance is thoroughly removed. This step helps to prevent contamination of the beer lines and maintains the integrity of the beer being served. Proper sanitation is vital in draft beer systems to ensure that flavors are not affected and that no unwanted chemicals remain in the lines or fittings.

This flushing process aids in achieving a clean and safe environment for serving beer, facilitating the desired flavors and aromas without interference from cleaning agents. It's a standard practice in the beer service industry to ensure cleanliness and prevent off-flavors, which is why this step is considered necessary.

The other options do not adequately address the importance of rinsing the lines to remove cleaner, which is why they are less appropriate in this context.