To me, the important thing is not the legs, but rather your body's switching to the guillotine-type orientation. Once you do that, it doesn't matter much whether you throw a leg over his leg (which is what I usually do), do nothing at all, or follow him all the way over into a mount. Just like an arm-in guillotine (which this really is a similar to, except that the arm is crossed, which it is for a great arm-in also), all those positions flow together, and having closed guard is unnecessary.
I typically like to throw a leg over just because if he tries to defend by going flat to his back, I will automatically follow into a full mount darce, rather than having to transition from N/S into a mounted darce or North/South choke.