I wrote the December LSAT, and for applying to Canadian law schools, I found there was no significant disadvantage at all. For American ones on the other hand....
I received my first offer of admission within 7 days of the school receiving my LSAT score. By the end of January, I had heard from all but one (I think I applied to 5 schools?) Remember, they will already have your entire file, so they could, theoretically, start reviewing it. An LSAT score is just, really, a number so it doesn't take that long to analyse!
It seems to me that schools in Canada make so few offers before January anyway, that writing in December is no big deal. Remember, if it were, they would suggest not writing then, but instead they only suggest not writing in Feb.
I think the bigger thing that should sway your decision on when to write is how much time you will have before the test to practice.