Medical exams for spousal sponsorships are done before application, as there is a requirement to include proof of the medical with the application. It should be one of the last things done before the application is ready, as it is valid for a year from the date it is done.
While there is no minimum income requirement for spousal sponsorships, family class applications can be denied for reasons of income under Section A39 of the Act. This section provides that if CIC is not convinced that adequate arrangements not including reliance on social welfare have been made for the needs of the applicant, and he/she is unwilling or unable to provide for their own needs, the application can be denied.
Therefore, if your wife is not making any money at this time, it is important for you to document in your application how your basic needs will be cared for. Is she job-hunting? Show proof. Will you live with family? Show proof. Does she already own a home? Show proof. Will you be living off savings? Show proof. Are you qualified to work in a trade or profession? Show proof. Stress the fact that you speak one or both (if true) of English and French. Show whatever you have that proves that you will have a roof over your head and you can provide for other basic needs. Doing these things can head off any Section A39 concerns that may arise due to her current income situation.