If you are planning a 10 day charter I would say absolutely. When you start to get shorter it depends on your plans and who you are taking. I could spend a entire week in the USVI quite happily. If you have newbies you want to show all the BVI sites things change. My ferry experiences the last few years were horrid but this last year all 3 trips went well and the ferry companies seemed to be trying. I would try for a 10 day charter and spend 3 days exploring St John. 6 cabin boats may be limited in the USVI so I would book early!
G