canadian flight plan advice for a U.S. pilot

Morning everyone!
I'm wondering if I can pick your brains regarding flight plan filing
for US/Canada flights. I recently made a trip from Boeing Field(KBFI)
up to Victoria BC (CYYJ) to learn about the procedures involved in
flying to Canada and back(I'm a U.S pilot based at KBFI). I took my
instructor along since it was my first time, since he has made the trip
numberous times and had great advice. Soooo... Let's asume you fly IFR-
you call up the FSS, file your U.S. Flight plan and make a great
landing at Victoria. For the return trip later that day, is it required
to file a Canadian Flight plan with their FSS ( I think they call them
FICs up there,not sure)? My instructor had me File the CYYJ to KBFI leg
at the same time I filed my Flight plan to Canada, so that it was
already there waiting for me on Clearance Delivery(I never had to call
the FIC). That seems so much easier- Is it OK to file both flight plans
in the US?
FYI, I'm leaving out the customs notification and procedures, as I take
care of those seperately. Just wondering about the flight plan filing.
Also, I've read the Nav Canada and AOPA websites- they seem to indicate
that you must file the Canada international flight plan for the trip
back. So,Did I get lucky filing a US flight plan for the trip back or
is the FSS just filing ICAO for me and I don't know it? Anyone with
experience in this, please give me your thoughts!
Thanks much!

Jamie