I'm a pretty big fan of Stargate SG-1 and when it was on the air, I was actually living in the city where the show was supposed to take place. Once in a while, I'd catch Amanda Tapping or Michael Shanks saying something and I'd immediately think to myself "Oh yeah, they're Canadian! The director should have caught that pronunciation. We wouldn't say it that way in Colorado." But it was rare, actually. So I'd say that they're using their real accents, with the possible exception of making sure to pronounce "out", "couch" and "mom" the (U.S.) American way.
Some of the tipoffs that those two actors are Canadian that come to mind are:
- sorry - the two actors I mentioned above, and some of my cousins who live in Toronto pronounce it "Soe ree" with a long O. Here, it's pronounced "Saw ree."
- project, the noun not the verb. Canadians (again, maybe not all of Canada, but those two actors, and the family I have in Toronto) say "Proe ject" with a long O. Here it is pronounced "praw ject."
But the biggest giveaway is when they use terms that we don't use here in the Western US. I remember Michael Shanks in an interview saying "my bum" when referring to his posterior. We would say "my butt" or "my behind." And in an episode I happened to watch just recently, when Daniel Jackson visited Atlantis, he said "I think I had her in Grade 5." That made me think the script writer was Canadian. Canadians will say "Grade 1, Grade 3, Grade 10" but folks in the U.S. say "First Grade, Third Grade, Sophomore year of high school."