When I had my interview, at the end the Lt. asked me if I had a preference in where I wanted to be posted. I asked what my options were (Infantry) he said Edmonton, Petawawa or Gagetown, I said, which ever will get me in the quickest. The Lt. then said not to put any preference down, because he said if for example I put Petawawa as my preference, and a job slot for Infantry opened up in Gagetown, I would not even be considered for the slot in Gagetown because Petawawa was my preference. So I didn't choose a preference at all.
The best advice I can give you is just talk to your recruiting center, and ask the person who interviews you.