Are you applying inland or outland? It's recommended to do your medical upfront and send the paperwork with your application, to avoid any delays in your processing. If you are applying outland, it's a requirement on the checklist so you must send it. Sometimes, however, they will take longer to process your application and your medical will expire in the meantime (I believe it's good for 1 year). Then you have to choose - sometimes they don't ask for a new medical, but they often do. You can wait until they ask for it or send a new one when you know the old one has expired.
I waited until they requested a new medical, and it only delayed the finalization of our application by a month (they give you 30 days to complete it when they request a new one, then look at your application again once the 30 days are up). It wasn't a big deal, though we did have to pay twice.