The medical is valid for 12 months from the day it's completed, so that's the only real issue in terms of the timing. When your wife has her medical done, the DMP will give her a copy of the medical form (just a receipt basically...not actual results or details), which you include with your application when you send it in. The DMP will then send in the medical to the visa office you specified on the form where they will keep it on file. Once you get past the initial sponsorship stage and Mississauga forward the application to your local visa office, they'll use your medical receipt to match it up against the medical the DMP sent in.
So really the timing isn't an issue, except from the point of view that if your application takes a long time to process, that 12 months could run out and she may be required to do another medical before the visa is issued. If it doesn't run out, her visa will only be valid until the 12 months is up. So based on your post, and assuming the application is approved within normal timelines (Buffalo currently takes 4-9 months), her visa would be valid until June 17th 2010 (i.e. that's when she would have to actually "land" in Canada to become a PR). Most people wait until they're almost ready to send their application, to give themselves almost a full 12 months for the application to be processed.
If your case is a slower one and takes 9 months in Buffalo, plus 1 month in Mississauga to complete the sponsorship portion, 10 months would be up before you have the passport returned with the visa. So doing the medical 2 months before sending in the application would mean she'd have to do another medical before they issue the visa. My advice is talk to your DMP about delaying when your wife actually does the medical (this is the important date as far as I'm aware, not when the DMP sends it in) until you're just about ready to send in your application to give yourself maximum time for the medical to remain valid.
My lawyer told me a month.