Different viruses can be most transmissible (able to spread) at different times. For instance, influenza (flu) is most transmissible at or just before the onset of symptoms. Whereas SARS is most transmissible at 7-10 days after symptom onset.
A team from Wuhan China created models based on data from two different studies of patients with COVID-19.

The researchers found that the highest viral loads on throat swabs collected were when symptoms started. The loads rapidly declined during the next 7 days. They concluded that about 44% of secondary cases among people in close contact (households) were infected in the 2 days prior to the onset of symptoms. Now this is based on modeling since no samples were collected in patients prior to onset of symptoms.

But if this is correct, then aggressive tracing of contacts will be even more critical than quarantining!

References: He X et L. Temporal dynamics in viral shedding and transmissibility of COVID-19. Nat Med 2020 April 15; [e-pub]. (https://doi.org/10.1038/s41591-020-0869-5)