[Crossposted from forkni-l. Backdated. In reply to a post noticing that Nick was the "circles" and Schanke the "crosses" in the tic-tac-toe game in the first-season episode "Dying to Know You," and that perhaps the "circles" were more "harmless" for Nick.]

Oh, what a wonderful observation!  I'd never noticed that, and it is just so much fun!  Where I live, we usually say "X"s and "O"s in tic-tac-toe, not "crosses" and "circles," so perhaps that's why I've missed the association.  But Nick wouldn't miss it, would he?  Especially if Schanke were drawing his "X"s as "+"s. 

What is the usual Canadian expression for the markings in tic-tac-toe?

Almost all tic-tac-toe games between adults tie, in my experience.  It's very easy to maneuver your opponent into a tie, and very hard to beat someone who knows how to maneuver a tie, if they're paying the least attention.  I think I'm going to interpret Nick and Schanke being evenly matched in tic-tac-toe as a symbolic affirmation of their partnership being one of equals well-matched, if very different.