Yes, it does depend on the time of year. When is it that you are going, again?
However, even when crowds are high, you can generally walk right onto whatever you hit first, if you are near the front of the entrance line and you walk over there briskly. Nemo is the possible exception-- people tend to run for it, and if you're not in the first wave, you might wait 20-30 minutes for it even if you arrive at the queue within 10 minutes of opening.
There's some recommended orders out there to do the rides in, depending on what you want to do. Let's assume you'll save Nemo to do last thing at night or on a different MM day if you have more than one (because it can eat up half or more of your MM hour).
Get to the gates at least 20 minutes early-- 30 is even better. That will put you near the front of the line at the turnstiles.
If you're going to do the Fantasyland rides (not just for little kids! I heart Peter Pan and Toady!

) Do Peter Pan first, always first.

That one builds up a line very quickly, does not have FP, and the line generally stays consistently long all day. Other ones that are slow loaders and tend to stay busy: Dumbo, Alice, Matterhorn, Teacups, Toad. Then hit Snow White, Pinocchio and the Carrousel (these are kind of "second string" rides for me; we often skip them). Then go ride Buzz and try to get onto Space Mountain before the regular crowds hit-- if they've already hit and the line is up more than you want to wait, just grab the FP for later.
The other thing, about general crowd volume-- it looks like zillions of people waiting at the gates on a busy weekend MM. But once they all get inside, it's pretty spread out for that first hour, especially compared to the midday crush. It doesn't seem crowded, that first hour.

Magic Mornings rock.